Platinized semiconductor particles

Almost all of the hitherto observed photoreactions can be rationalized by the simplified mechanistic scheme as depicted in Figure 1 for a platinized semiconductor particle. It is noted that platinization in most but not all cases is required in order to obtain acceptable reaction rates. 

Fig. 5. Schematic representation of a photocatalytic reaction on a platinized semiconductor particle.

Ohtani B, Osaki H, Nishimoto S, Kagiya T (1986) A novel photocatalytic process of amine N-alkylation by platinized semiconductor particles suspended in alcohols. J Am Chem Soc 108(2) 308-310... 

Fig.6. Photocatalytic cleavage of H2S over a platinized composite sulfide semiconductor particle with a heterojunction.

Aqueous pools of reversed micelles have been fruitfully employed for the in situ generation of semiconductor particles. The first publication in this area described the formation of CdS in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) aggregates in isooctane [611]. The preparation involved the addition of aqueous CdCl or Cd(N03)2 to isooctane solutions of AOT. Exposure to controlled ammeters of the CdS particles formed. Irradiation of degassed, AOT-reversed-micelle-entrapped, platinized CdS by visible light (450-W Xenon lamp X > 350 nm) in the presence of thiophenol (PhSH) resulted in sustained hydrogen formation. Sacrificial electron transfer occurred from thiophenol to positive holes in the colloidal CdS and, consequently, diminished undesirable electron-hole recombinations (Fig. 101) [611]. 

The rate of flow of electrons from such a charged particle depends on the availability of an accessible site for this transfer. Although it is known that lattice defects provide such sites and that conduction band electrons can trickle down through solid dislocation levels reduction sites for electron accumulation are usually provided by metallization of the semiconductor particle. This can be achieved through photo-platinization or by a number of vapor transfer techniques and the principles relevant to hydrogen evolution on such platinized surfaces have been delineated by Heller The existence of such sites will thus control whether single or multiple electron transfer events can actually take place under steady state illumination. 

It has been known for some time that irradiation of platinized CdS powders or colloids in aqueous solution containing an electron donor leads to formation of H2, in competition with dissolution of the semiconductor. Several groups have continued work in this field and HjS has been identified as a convenient electron donor. It has been shown that the photochemical activity of platinized CdS is improved markedly if the surface is etched before use. The presence of sulphite ions prevents accumulation of the S2 ions which inhibit H2 formation so that this system provides an efficient method for removal of H2S. Incorporating the platinized CdS particles in vesicles does not restrict H2 formation. 

As already mentioned before, mainly irreversible reactions with organic compounds have been investigated at semiconductor particles. When organic molecules, for example alcohols, are oxidized by hole transfer, O2 usually acts as an electron acceptor or in the case of platinized particles, protons or H2O are reduced. A whole sequence of reaction steps can occur, which are frequently difficult to analyze because cross-reactions may also be possible at particles and a new product could be formed. Concerning the primary electron and hole transfer, certainly there should be no difference between particles and compact electrodes. Since sites at which reduction and oxidation occur are adjacent at a particle, the final product may be different. An interesting example is the photo-Kolbe reaction, studied for Ti02 electrodes and for Pt-loaded particles. Ethane at extended electrodes and methane at Pt/Ti02 particles have been found as reaction products upon photo-oxidation of acetic acid [56, 57]. The mechanism was explained by Kraeutler et al. as follows. 

Water and isopropyl alcohol serve as electron donors in the 1 and 2 reaction, respectively. In this way traces of gold [56], silver [57], mercury [58], platinum [59] and other metals can be removed from solutions. Same procedure was used to purposely deposit noble metal "islands" as catalyst onto semiconductor particles (see above), e.g., to prepare platinized Ti02 suspensions by illuminating Ti02 particles in the H2PtCl6 solution. 

The polymeric perfluorinated sulfonic acid has been used as a matrix for a system which combines semiconductor CdS crystallites and a Pt hydrogen-evolution catalyst In a photocatalytlc hydrogen generator (73-74). Upon photolysis of the platinized CdS particles In the presence of a sacrificial electron donor, Na.S, the production of hydrogen gas by water reduction was observed. The number of moles of produced with a typical NAFION/(MS system exceeds the moles of CdS present by a factor greater than 100. 

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. 

Figure 10. Scheme of water photoelectrolysis on a particle of semiconductor (platinized SrTi03) suspension in an aqueous solution. 

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