Semiconductor powder separation

Similar to the molecular photosensitizers described above, solid semiconductor materials can absorb photons and convert light into electrical energy capable of reducing C02. In solution, a semiconductor will absorb light, and the electric field created at the solid-liquid interface effects the separation of photo-excited electron-hole pairs. The electrons can then carry out an interfacial reduction reaction at one site, while the holes can perform an interfacial oxidation at a separate site. In the following sections, details will be provided of the reduction of C02 at both bulk semiconductor electrodes that resemble their metal electrode counterparts, and semiconductor powders and colloids that approach the molecular length scale. Further information on semiconductor systems for C02 reduction is available in several excellent reviews [8, 44, 104, 105],... 

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. 

Y. Nosaka Y. Ishizuka H. Miyama, Separation mechanism of a photo-induced electron-hole pair in metal-loaded semiconductor powders. Ber. Bunsenges. Phys. Chem. 1986, 90, 1199-1204. 

The first commercial application of photocatalysts has started to clean our environment by Ti02 powders and films. In order to utilize photocatalysts for solar energy conversion, sensitization of large bandgap semiconductors is important. The most difficult task for an artificial photosynthetic system is to establish visible light-induced charge separation with minimum back charge recombination. 

Figure 2. Electron-hole separation on a metallized (M) semiconductor (SC) powder.

Other Semiconductor-Ruthenium Complex Systems. Taqui Khan et al. used a ruthenium(III)-EDTA-bipyridyl complex as a photosensitizer in a Pt-Ti02 semiconductor particulate system [109]. The Ti02 powder was loaded with platinum by the procedure of Erbs et al. [110]. Here 50 mg of the Pt-Ti02 powder was stirred in 10 mL of 0.001 M K[Ru(EDTA)(bipy)] 3 to deposit the ruthenium complex on the Ti02 surface. The 3-treated powder was separated by centrifugation and dried at room temperature. 

The semiconductor catalyst is generally used as powder suspended in a hquid medium. The inconvenience of this approach at large scale is the catalyst-recovering step from the solution at the end of operation. The sohd-liquid separation is an extremely important issue for the development of the photocatalytic technology indeed, the best possible recovery of particles must be ensured, in order to prevent their wash out and consequent decrease of their amount in the reactor system. 

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