Colloidal semiconductor photocatalysts

Kamat PV, Dimitrijevic NM (1990) Colloidal semiconductors as photocatalysts for solar energy conversion. Sol Energy 44 83-98... 

Semiconductor photocatalysts in a form of colloids, powders, porous granules, thin films or bulk solids including single crystals (used in model studies) provide both liquid phase and gas phase transformations. Comprehensive reviews in this field can be found in monographs [4] (Chapters by N.S.Lewis and M.L.Rosenbluth M.Gratzel M.Schiavello and A.Sclafani P.Pichat and J.-M.Herrmann G.A.Somorjai T.Sakata H.Tributsch M.A.Fox H.Al-Ekabi and N.Serpone D.F.Ollis, E.Pelizzetti and N.Serpone) [8] (Chapter by Yu.A.Gruzdkov, E.N.Savinov and V.N.Parmon) and [3]. 

We have already seen that photoactive clusters, e.g. CdS, can be introduced into vesicles and BLMs (Sect. 5.2 and 5.3). Similar support interactions are possible with both inorganic and organic polymeric supports. Photoactive colloidal semiconductor clusters can be introduced, for example, into cellulose [164], porous Vycor [165], zeolites [166], or ion exchange resins [167]. The polymer matrix can thus influence the efficiencies of photoinduced electron transfer by controlling access to the included photocatalyst or by limiting the size of the catalytic particle in parallel to the effects observed in polymerized vesicles. As in bilayer systems,... 

Colloidal semiconductor particles have been found to act as heterogeneous photocatalysts in a number of environmentally important reactions. 

When deployed on-line, the semiconductor photocatalyst may be required to photoreduce more than one type of actinide metal ion simultaneously. Figure 9 shows the effect of illuminating U(VI) with light of wavelength 350 nm in the presence of colloidal SnCh, nitric acid (pH 0) and ethanol as an electron scavenger for the semiconductor photocatalyst and Ce(IV) as a non-radioactive, thermodynamic analogue for Pu(IV). Comparison of the data in Fig. 9 with the data recorded under similar conditions as shown in Fig. 7 indicates that the presence of Ce(IV) has no effect on the rate of photocatalysed reduction of U(VI) to U(IV). Furthermore, spectroscopic analysis indicates that virtually all of the Ce(IV) has been reduced to Ce(III) over the same timescale, suggesting that the simultaneous photocatalysed reduction of two or more different types of (actinide) metal ion can be accomplished with no loss of yield for either reaction. 

Hydrazone cyclization and hydroalkylation [138-140] are rare examples of reactions conducted on a preparative scale, since the products were isolated in milligram amounts and not just identified in solution. As already mentioned in Section 6.2.5, photocorrosion of the semiconductor photocatalyst often prevents its use in preparative chemistry. This is very true also for colloidal semiconductors although the pseudo-homogeneous nature of their solutions allows one to conduct classical mechanistic investigations, until now they were too labile to be used in preparative chemistry [107, 141, 142]. In contrast to the above-mentioned reactions, in recent years we have isolated novel compounds on a gram-scale employing photostable zinc and cadmium sulfide powders as photocatalysts [97, 107, 143-145]. During this work we found also a new reaction type which was classified as semiconductor photocatalysis type B [45]. In contrast to type A reactions, where at least one oxidized and one reduced product is formed, type B reactions afford only one unique product, i.e., the semiconductor catalyzes a photoaddition reaction (see below). 

Passive films (corrosion) Photoredox processes with colloidal semiconductor particles as photocatalyst (e.g., degradation of refractory organic substances) Photoelectrochemistry (e.g., photoredox processes at semiconductor electrodes)... 

The use of semiconductor colloids as photocatalyst for a variety of chemical reactions, due to their peculiar optoelectronic photocatalytic properties, is well stated in the recent literatures [50-52]. The effectiveness of the photodegradation processes has already been tested for different types of matrices and results have been encouraging, as the literature reports on a large number of successes in the... 

It is possible that colloidal photochemistry will provide a new approach to prebiotic syntheses. The work described previously on redox reactions at colloidal ZnS semiconductor particles has been carried on successfully by S. T. Martin and co-workers, who studied reduction of CO2 to formate under UV irradiation in the aqueous phase. ZnS acts as a photocatalyst in the presence of a sulphur hole scavenger oxidation of formate to CO2 occurs in the absence of a hole scavenger. The quantum efficiency for the formate synthesis is 10% at pH 6.3 acetate and propionate were also formed. The authors assume that the primeval ocean contained semiconducting particles, at the surface of which photochemical syntheses could take place (Zhang et al 2007). 

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. 

Colloidal solutions of semiconductor particles are of great interest mainly as photocatalysts of various processes. Of special interest are the so-called Q-particles of semiconductors (for CdS with the size 2R < 50 A), some of their properties being considerably different from... 

Electrophoretic deposition (EPD) is a colloidal process in which the charged colloidal particles are driven by a dc electric field to deposit on a substrate, forming a condensed film. This process is a combination of electrophoresis and deposition (Sarkar and Nicholson 1996). It has a long history and the first application was in 1927 for Th02 and tungsten deposition on a platinum cathode. Recently, photocatalyst semiconductor nanoparticles/microparticles have also been assembled by this... 

The study of heterogeneous photosystems, involving u.v. or visible light irradiation of aqueous suspensions of semiconductor powders or colloids, has increased steadily over the past five years. Many studies are aimed at development of systems capable of cyclic water cleavage, although, as mentioned earlier, Ti02 doped with iron functions as a photocatalyst for reduction of N2 to The most popular semiconductor materials continue to be TiOj... 

Colloidal or powdered semiconductors can be incorporated into polymer films, which then behave as photocatalysts. This should lead to some interesting applications as photocatalysts in the form of films. CdS particles were embedded in a polyurethane film and photolysis processes studied Nafion film was used to disperse CdS Nafion films were soaked in Cd(N03)2 aqueous solution and then treated with H2S gas to give CdS-dispersed films A Pt catalyst was also incorporated into the Nafion... 

Zinc oxide has similar photocatalytic properties, but it has satisfactory chemical stability only in neutral or alkaline media, whereas in acidic media it dissolves [10, 11], Tin oxide is also a well-known photocatalyst as well as a preferred basic material for sensors [12, 13]. These may be prepared using classical colloid chemical methods, i.e. by converting the metal salts of the semiconductor into metal hydroxides by hydrolysis, followed by... 

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