Semiconductor dispersions

Interfacial electron transfer in colloidal metal and semiconductor dispersions and photodecomposition of water. K. Kalyanasundaram, M. Gratzel and E. Pelizzelti. Coord. Chem. Rev., 1986, 69, 57 (338). 

Darwent, J.R., H2 production photosensitized by aqueous semiconductor dispersions, /. Chem. Soc., Faraday Trans. II, 77,1703,1981. 

Serpone, N., Borgarello, E., Giatzel, M. 1984. Visible-light induced generation of hydrogen from HjS in mixed semiconductor dispersions improved efficiency through inter-particle electron transfer. Chem Commun 6 342-344. 

Separate nanometer- to micron-sized colloidal semiconductors dispersed in solutions they are usually stabilized by polyions or polymers. 

Figure 1. Basic features of sacrificial water reduction systems. (A) Homogeneous solution, with sensitizer, S, electron relay, R, sacrificial electron donor, D, and metal catalyst. (B) Catalyst-coated colloidal semiconductor dispersion, obviating the need for electron relay.

Barbeni, M., Pramauro, E., Pelizzetti, E., Borgarello, E., Serpone, N., Jamieson, M.A. (1986) Photochemical degradation of chlorinated dioxins, biphenyls, phenols and benzene on semiconductor dispersion. Chemosphere 15, 1913-1916. 

In recent years there has been interest in using semiconductor dispersions in the form of colloidal particles instead of macroscopic electrodes70. The area/volume ratio is clearly larger, which gives increased yields. Colloidal semiconductors investigated are principally n-Ti02 and cadmium sulphide with adatoms (surface states) of platinum. The particles have to function simultaneously as cathode and anode. Figure 12.24 shows their mode of operation schematically for reduction of A, aided by oxidation of a dye, D. 

H2 Production Photosensitized by Aqueous Semiconductor Dispersions. As in Entry 6 but a fuller study by the same group. 494... 

In particular, the potential distribution within a spherical semiconductor particle could be derived [97] using a linearized Poisson-Boltzmann equation. As discussed in [69], two limiting cases are of special importance for light-induced electron transfer in semiconductor dispersions. For large particles, the total potential drop within the particle is... 

Gratzel M. and Frank A. J. (1982), Interfacial electron-transfer reactions in colloidal semiconductor dispersions—kinetic analysis , J. Phys. Chem. 86, 2964-2967. 

Moser, J. and Gratzel, M., Light-induced electron transfer in colloidal semiconductor dispersions Single vs. dielectronic reduction of acceptors by conduction-band electrons, J. Am. Chem. Soc., 105, 6547, 1983. 

Generation of photocurrent at the semiconductor/electrolyte interface upon its illumination makes it possible to carry out photoelectrochemical reactions which can be used either for chemical fuel production, or purification of waters. Principles of operation of electrochemical cells with semiconductor electrodes for solar energy conversion to electrical and chemical energy are formulated. Most efficient cells for electricity and hydrogen production are surveyed. Certain processes for photo-destruction of pollutants, recovery of metals, etc. with making use of semiconductor dispersions are briefly discussed. 

In this paper general principles of operation will be briefly outlined for PEC cells, and most important systems surveyed. (For detailed presentation of the semiconductor photoelectrochemistry, see, e.g., [2], and for description of PEC cells at greater length, see [3].) In the last Section, some works on the semiconductor dispersions for purifying wastewaters from pollutants will be reviewed. 

In recent years great interest has aroused in photoelectrochemical behaviour of the semiconductor dispersions, e g., suspensions, colloids, small particles embedded in membranes and vesicles, etc. Unlike PEC cells, they do not provide for special separation of products of the photoelectrochemical process. However, the microheterogeneous systems are superior in electrocatalytic activity (due to their giant surface area). 

Already at an early stage of the research in the semiconductor dispersions, attempts have been made to carry out water splitting, CO2 reduction, etc., in other words, the same photoelectrochemical processes as in the macroscopic PEC cells. The results obtained are summarized in [51-55]. We shall confine ourselves, however, to the processes that might underly some methods of purification, eg., of waste waters etc. These processes are stimulated by electrons and holes produced in the particles by light. As only one type of the current carriers is consumed in the "useful" reaction, measures should be taken to remove the other type from the particle in order to preserve its electroneutrality and sustain the process. For this purpose a sacrificial electron donor (or acceptor) is to be introduced into the electrolyte solution. Often it is the solvent that plays sacrifice. Some examples are listed below. 

PEC cells for solar-to-electrical energy conversion PEC cells for solar-to-chemical energy conversion Semiconductor dispersions for providing cleaner environments Concluding remarks References... 

U. Kolle, J. Moser, M. Gratzel, Dynamics of interfacial charge-transfer reactions in semiconductor dispersions - reduction of cobaltoceniumdicarboxylate in colloidal Ti02, Inorg. Chem. 1985,24(14), 2253-2258. 

In the photochemical conversion of solar energy, it may prove possible to use colloidal semiconductors dispersed in polymerised vesicles. 

Water purification. Semiconductor systems, such as particulate Ti02> have already demonstrated the possibility of oxidation of contaminants such as C, S02> and organics in aqeous solutions in the presence of oxygen. Irradiated semiconductors are very potent oxidizing agents, since hydroxyl radical is produced in some systems. Semiconductor dispersions should also be useful for disinfection and the oxidation of even very resistant organic molecules. 

These types of measurements have been confirmed for various semiconductor dispersions (27,131-133) including Ti02, In203, Sn02, CdS, WO3, and Fc203, and demonstrated that the... 

Photocatalysis over irradiated semiconductor dispersions provides a method that can lead to a highly effective, spatially controlled oxidation and reduction of organic and... 

Hidaka, H. Kubota, H. Graetzel, M. and Serpone, N. (1985) Photodegradation of surfactants I.Degradation of sodium dodecylbenzene sulfonate in aqueous semiconductor dispersions . Nouveau J. de Chimie 9, 67-69. 

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