Semiconductor powder surface

In a series of transition metal oxide semiconductor powders, photochemical activity in the decarboxylation of oxalic acid was controlled by surface properties and the presence of recombination centers, which in turn depended on the preparation method Similar effects have also been noted in the photodecarboxylation of pyruvic acid and formic acid... 

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. 

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. 

We have shown how the band structure of photoexcited semiconductor particles makes them effective oxidation catalysts. Because of the heterogeneous nature of the photoactivation, selective chemistry can ensue from preferential adsorption, from directed reactivity between adsorbed reactive intermediates, and from the restriction of ECE processes to one electron routes. The extension of these experiments to catalyze chemical reductions and to address heterogeneous redox reactions of biologically important molecules should be straightforward. In fact, the use of surface-modified powders coated with chiral polymers has recently been reputed to cause asymmetric induction at prochiral redox centers. As more semiconductor powders become routinely available, the importance of these photocatalysts to organic chemistry is bound to increase. 

One of the major problems encountered in this field concerns the extremely poor reproducibility of the photochemical properties of semiconductor powders and colloids prepared in different laboratories. Often, the materials are badly characterized and any pretreatment is inadequately described. The source, crystal structure, surface contamination, and pretreatment are all critical in determination of the photoactivity of a semiconductor, and until attention is given to these factors the subject will suffer from irreproduci-bUity. These problems become magnified when the semiconductor is doped or coated with a noble metal. 

Even without deposition of a metal island, wide band-gap semiconductor powders often maintain photoactivity, as long as the rates or the positions of the oxidative and reductive half reactions can be separated. Photoelectrochemical conversion on untreated surfaces also remains efficient if either the oxidation or reduction half reaction can take place readily on the dark semiconductor upon application of an appropriate potential. Metalization of the semiconductor photocatalyst will be essential for some redox couples, whereas, for others, platinization will have nearly no effect. Furthermore, because the oxidation and reduction sites on an irradiated particle are very close to each other, secondary chemical reactions can often occur readily, as the oxidized and reduced species migrate toward each other, leading either to interesting net reactions or, unfortunately, sometimes to undesired side reactions. 

For the influence of the specific surface area of the semiconductor powder on the rate of product formation, two opposite effects are of major importance [81]. One is concerned with the rate of electron-hole recombination which increases linearly with surface area, and accordingly the reaction rate should decrease. The other is a linear increase in the reaction rate of the reactive electron-hole pair with the adsorbed substrates, which should increase product formation. It is therefore expected that, depending on the nature of semiconductor and substrates, the reaction rate, or increasing surface area. This is nicely reflected by the CdS/Pt-catalyzed photoreduction of water by a mixture of sodium sulfide and sulfite. The highest p values are observed with small surface areas and are constant up to 2 m g". From there a linear decrease to almost zero at a specific surface area of 6 m g" takes place. Upon further increase to 100 m g" this low quantum yield stays constant [82]. 

Kato, T. Loo, B. H. Yokomaku, M. Butsugan, Y Sim, K. Y Fujishima, A. Photoelectrochemical reduction of tetrazolium salts to formazans on surfaces of semiconductor powders in alcohol solutions. Spectrosc. Lett. 1995, 28, 849-859. 

Photocatalysis by semiconductor powders, most usually titanium dioxide, has been applied to the oxidation of several aromatics. In many such photocatalytic reactions, the key intermediates are hydroxyl radicals formed by oxidation of water. With rather good donors such as aromatic compounds, hole transfer on the excited semiconductor surface is a viable alternative in this case, the reaction of the radical cation of the substrate or further intermediates arising from it with oxygen or superoxide anion may have a role. Photocatalyzed oxidation of naphthalene yields 2-formyldrmamaldehyde and 1,4-naphthoquinone as the primary products (Eq. (45.16)), similarly to what occurs upon direct irradiation of naphthalene in water ... 

Photoreactions on ZnO powder in aqueous suspension and in contact with gases have often been studied during the last few decades, and only a few aspects of this work are reviewed here. For example, nitrous oxide and methyl iodide were found to decompose when brought into contact at 20 °C with the illuminated surface of ZnO and nitrate, indigo carmine and p-nitrosodimethylaniline were found to be reduced in aqueous suspensions ZnO is of special interest as it is one of the standard electrode materials in conventional semiconductor electrochemistry and photo-electrochemistry Colloidal ZnO has not been available until recently. It... 

Cadmium oxide, CdO, is a semiconductor with a band gap of 2.3 eV. Irradiation of CdO powder suspended in alkaline solution resulted in the formation of 0 when an electron acceptor such as ferricyanide was present in the solution. When RuOj was deposited onto the surface of the CdO particles the yield of decreased relative to naked CdO. In this respect CdO differs from Ti02 where RUO2 is mandatory if O2 evolution is to be observed Colloidal CdO has not been known until recently. It can... 

Platinum-loaded Ti02 systems can be considered as a short-circuited photo-electrochemical cell where the Ti02 semiconductor electrode and metal Pt counterelectrode are brought into contact [159]. Light irradiation can induce electron-hole (e -h +) pair formation and surface oxidation and also reduction reactions on each Pt/Ti02 particle (Figure 4.11). These powder-based systems lack the advantage of... 

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