Metallized semiconductor powder

Fig. 3. Redox Exchange on an Irradiated Metallized Semiconductor Powder... 

Water Photolysis by Metallized Semiconductor Powders 13.3.1 Gas-phase Water Photolysis by NaOH-coating... 

Besides the solution thickness on the catalyst and the concentration of deliquescent material, various other factors which may influence the yield of water photolysis over metallized semiconductor powders have been examined. 

A similar situation also is encountered in a miniaturized photoelectrochemical cell, i.e., on a metallized semiconductor powder, Figure 2. Here, the individual particle can be thought of as two electrochemical half cells which have eventually collapsed onto each other as the conductive wire connecting them became shorter and shorter. The oxidizing and reducing sites are thus found in close spatial proximity and the potential for subsequent chemical reaction between the initial oxidation and reduction products is excellent. In fact, so long as the respective rates of the oxidation and reduction half reactions differ appreciably, it may be unnecessary to metallize the semiconductor powder in order to... 

Figure 4. Charge trapping on a metalized semiconductor powder, as an analog to a short-circuited electrochemical cell.

The photo-Kolbe reaction is the decarboxylation of carboxylic acids at tow voltage under irradiation at semiconductor anodes (TiO ), that are partially doped with metals, e.g. platinum [343, 344]. On semiconductor powders the dominant product is a hydrocarbon by substitution of the carboxylate group for hydrogen (Eq. 41), whereas on an n-TiOj single crystal in the oxidation of acetic acid the formation of ethane besides methane could be observed [345, 346]. Dependent on the kind of semiconductor, the adsorbed metal, and the pH of the solution the extent of alkyl coupling versus reduction to the hydrocarbon can be controlled to some extent [346]. The intermediacy of alkyl radicals has been demonstrated by ESR-spectroscopy [347], that of the alkyl anion by deuterium incorporation [344]. With vicinal diacids the mono- or bisdecarboxylation can be controlled by the light flux [348]. Adipic acid yielded butane [349] with levulinic acid the products of decarboxylation, methyl ethyl-... 

SSMS can be classified among the milliprobe techniques (Figure 8.3), i.e. it is a unique link between microprobe techniques and macroanalytical methods that are characterised by poor lateral and in-depth resolutions (as in OES), or that have no lateral resolution whatsoever (as in NAA). Also, the achievable precision and accuracy are poor, because of the irreproducible behaviour of the r.f. spark. Whereas analysis of metals, semiconductors and minerals is relatively simple and the procedures have become standardised, the analysis of nonconducting materials is more complex and generally requires addition of a conducting powder (e.g. graphite) to the sample [359]. Detection limits are affected by the dilution, and trace contamination from the added components is possible. These problems can be overcome by the use of lasers [360]. Coupled with isotope dilution, a precision of 5% can be attained for SSMS. 

Sayama, K., Arakawa, H. 1992b. Remarkable effect of Na CO, addition on photodecomposition of liquid water into H, and O, from suspension of semiconductor powder loaded with various metals. Chem Lett 2 253-256. 

Baba R, Nakabayashi S, Fujishima A, Honda K (1985) Investigation of the mechanism of hydrogen evolution during photocatalytic water decomposition on metal-loaded semiconductor powders J Phys Chem 89 1902-1905... 

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... 

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],... 

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. 

The mechanistic principles which enable a semiconductor powder, suspended in a solution of substrates, to catalyze a photoreaction originate in the field of photoelectrochemistry. For a critical discussion of the basic assumptions made in the deduction of the resulting theories and their relevance to catalysis, the reader is referred to recent review articles [33-35]. The use of a metallized semiconductor particle as a kind of short-circuit micro-photoelectrochemical cell was introduced through the work of Kreutler and Bard on the photo-Kolbe reaction [36]. Differences from the macroelectrodes employed in more conventional photoelectrochemistry have been discussed [34, 37-40]. 

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 platination of semiconductor powders is a method for producing semiconductor-metal type photochemical diodes with an energy level scheme as shown in Fig. 11.14a. This was demonstrated for the first time with platinized Ti02 powders which showed excellent photocatalytic activity for the photodecarboxylation of acetate (the photo-Kolbe reaction), a process which has already been discussed in detail in Section 9.2.3 [80]. Various attempts have also been made to photocleave water by using semiconductor powders on which a catalyst such as Pt or RUO2 has been deposited. The relevant experiments usually failed, either because the semiconductor was... 

Antimony Antimony BIxk Antimony, elemental Antimony, metallic Antimony powder Antimony, regulus Antymon Cl 77050 EINECS 231-146-5 HSDB 508 Regulus of antimony Stibium Stibium metallicum UN2871. Hardening alloy for lead, bearing metal, type metal, solder, collapsible tubes and foil, sheet and pipe, semiconductor technology, pyrotechnics. mpn630" bp = 1635" d =... 

BET measurements were not feasible for aluminum and silicon metal samples because their surface areas were below detection limits. The specific surface area of aluminum powder was obtained by correcting the Kozeny value with the average BET/Kozeny ratio of 3.5 1.6. Surface areas of aluminum metal foil and silicon metal semiconductors were obtained by direct measurement, multiplied by a roughness factor of two, which was estimated on the basis of geometric considerations of etched metallic surfaces. 

The phenomenon of emission of light by substances due to application of AC or DC electric field is called electroluminescence (EL). It has been observed in a number of materials in form of powder, thin films, single crystals, p-n junction and metal-semiconductor and metal-insulator-semiconductor stractures, and so on. The phenomenon of EL can be considered to be comprised of three sequential processes (i) excitation, (ii) transfer of energy from site of excitation to that of emission, and (iii) recombination. The EL involves the exciation of luminescence as a result of existence of an apphed electric potential difference across the phosphor. The EL properties of nanomaterials and nanocomposites can be significantly controlled by changing the size of the particles. 

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