Irradiated semiconductor surfaces

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) ... 

That enone formation accompanies allylic alcohol can be explained by the known oxidizability of alcohols on irradiated semiconductor surfaces (288) or by dehydration of an intermediate allylic hydroperoxide. 

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... 

This review was prepared as part of our research program on photoinduced electron transfer in functionalized polymers and at irradiated semiconductor surfaces. That research is funded by the Office of Basic Energy Sciences, Fundamental Interactions Branch, of the Chemistry Division of the U.S. Department of Energy. 

Because radical ions are known to react through a number of possible reaction pathways, [59], photoelectrochemistry provides, in principle, a possible method for choosing among competitive routes. The formation of a cation radical by hole trapping by an organic donor on an irradiated semiconductor surface often gives rise to products different from those obtained on a poised metal electrode or those derived from the same cation radical when produced in homogeneous solution (Eq. 5) [60]. 

Fox MA (2001) S3mthetic applications of photocatalytic oxidation and reduction reactions of organic reactants on irradiated semiconductors surfaces. In Balzani V (ed) Electron transfer in chemistry, vol I. Wiley-VCH, Weinhetm, p 271 Li Y, Wang L (1997) Stud Surf Sci Catal 103 391 Ohtani B (1994) Trends Photochem Photobiol 3 531... 

Inferences that oxidation takes place on the photocatalyst s surface have been made (67). No such conclusions can be drawn. Similar observations have been made in homogeneous media if a bimolecular reaction between two reactants is assumed. A Langmuir-type behavior is no guarantee of a surface occurring process. A rigorous treatment (68) of the kinetics involved in the photocataly2ed oxidations of organic substrates on an irradiated semiconductor has confirmed this. 

D.M. Zehner, Surface Studies of Pulsed Laser Irradiated Semiconductors D.H. Lowndes, Pulsed Beam Processing of Gallium Arsenide R.B. James, Pulsed C02 Laser Annealing of Semiconductors R. T. Young and R.F. Wood, Applications of Pulsed Laser Processing... 

Thus, the hydrophilic nature of the semiconductor surface is increased on irradiation. 

Since photoexcited electron-hole pairs are formed only within a limited depth from the semiconductor surface to which the irradiating photons can penetrate, the photon-induced split of the Fermi level into the quasi-Fermi levels of electrons and holes occurs only in a surface layer of limited depth as shown in Fig. 10-2. 

On the other hand, oxide semiconductor materials such as ZnO and 2 have good stabilities under irradiation in solution. However, such stable oxide semiconductors cannot absorb visible light because of their wide band-gap character. Sensitization of wide-band-gap oxide semiconductor materials by photosensitizers, such as organic dyes which can absorb visible light, has been extensively studied in relation to the development of photography technology since the middle of the nineteenth century. In the sensitization process, dyes adsorbed onto the semiconductor surface absorb visible light and excited electrons of dyes are injected into the conduction band of the semiconductor. Dye-sensitized oxide semiconductor photoelectrodes have been used for PECs. 

We can infer that the band positions of the irradiated semiconductor are greatly influential in controlling the observed redox chemistry and that formation of radical ions produced by photocatalyzed single electron transfer across the semiconductor-electrolyte interface should be a primary mechanistic step in most such photocatalyzed reactions. Whether oxygenation, rearrangement, isomerization, or other consequences follow the initial electron transfer seem to be controlled, however, by surface effects. 

Photoexcitation of n-type semiconductors renders the surface highly activated toward electron transfer reactions. Capture of the photogenerated oxidizing equivalent (hole) by an adsorbed oxidizable organic molecule initiates a redox sequence which ultimately produces unique oxidation products. Furthermore, specific one electron routes can be observed on such irradiated surfaces. The irradiated semiconductor employed as a single crystalline electrode, as an amorphous powder, or as an optically transparent colloid, thus acts as both a reaction template and as a directed electron acceptor. Recent examples from our laboratory will be presented to illustrate the control of oxidative cleavage reactions which can be achieved with these heterogeneous photocatalysts. 

Although it is clear that photoinduced redox exchange can occur efficiently at the surface of an irradiated semiconductor powder, this redox chemistry will not find extensive use unless it provides access to new chemical transformations which are inaccessible with conventional reagents or to an improved selectivity in multifunctional molecules or in mixtures of reagents. 

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... 

With the semiconductor oxidation catalyst, however, the surface becomes activated only upon photoexcitation. At low light intensities, the possibility that many holes are formed in the valence band is remote, so that the irradiated semiconductor powder becomes an effective one-electron oxidant. Now if the same chemistry ensues on the photochemically activated TiC>2 surface, then the reaction will proceed as in the bottom route of eqn 9. Thus, the carboxy radical is formed, producing an alkyl radical after loss of carbon dioxide. Since the semiconductor cannot continue the oxidation after the first step, the radical persists, eventually recapturing the conduction band electron, either directly or through the intervention of an intermediate relay, perhaps superoxide. The resulting anion would be rapidly protonated to product. 

The colloidal or powder particle can be composed of either insulating, semiconductive, or conductive molecules. While only semiconductor particles are likely to be photoactive per se (by virtue of the energy gap between the filled valence band and the vacant conduction band), photoactivity of adsorbates can be mediated at the surface of other solids [18] which are often used themselves, or in conjunction with an irradiated semiconductor, as catalytic sites for alteration of kinetics of dark reactions initiated by photoexcitation. 

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. 

In typical investigations on the behavior of modified TiC>2 surfaces upon irradiation, measurements are carried out in acetonitrile containing 0.1 M UCIO4. Under these conditions, very fast electron injection into the semiconductor surface is observed. However, it has been noted that this injection process depends strongly on the lithium concentration of the contacting acetonitrile solution [10,11]. In the absence of lithium, no injection is observed. This is an important observation since this opens up the possibility of modulating the photophysical behavior of the interfacial supramolecular assembly by external manipulation of the conditions. In this section, this observation is discussed in more detail, and, in addition, the possibility to use the surface potential of the semiconductor surface as a driving force will be considered. 

After the adsorption of inorganic (02, 03, NO, N02, S02, CO, C02, etc.) or organic molecules onto the semiconductor surface and especially after further illumination of a sample prepared, different stable or relatively stable radicals are easily recorded by the EPR method. Several important systems in which charge separation created organic radicals were described in detail in Chapter 1 of this book. Some additional information concerning adsorbed pentane, methane, ethylene, benzene, methylbenzenes and m-dinitrobenzene can be found in publications [41, 60, 69-74]. Further, we will shortly discuss some structural features of paramagnetic centers formed under chemical activation or irradiation of the adsorbed oxygen or NxOy molecules. 

Investigations of metals (Ag [28-30], Au [31, 33], Pt [18, 22, 30], Pd [11], Cu, Fig [41] etc.) photoreduction at surfaces of porous samples and colloidal particles of Ti02 shows, that in such systems metal is deposited on the semiconductor surface as separate particles of subnanometric - nanometric size. Such metal particles have ohmic contact with semiconductor surface [11, 24, 32] and developed electronic structure [24, 28-32], So, we concluded that photocatalytic nickel(II) reduction takes place at irradiation of suspensions, containing mesoporous Ti02, Ni2+ and ethanol, this process resulting in the... 

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