Colloidal Semiconductor Systems

Duonghung D, Ramsden J, Gratzel M (1982) Dynamics of interfacial electron-transfer processes in colloidal semiconductor systems. J Am Chem Soc 104 2977-2985... 

Kamat, P.V., Interfacial charge transfer processes in colloidal semiconductor systems, Prog. React. Kinet., 19,277,1994. 

Graetzel, M. Dynamics of interfacial electron transfer reactions in colloidal semiconductor systems and water cleavage by visible light, Nato Asi Ser., Ser. C 1986, 174, 91. 

This review is a discussion of the kinetic modelling of the photoelectrochemistry of colloidal semiconductor systems. This area is currently attracting significant attention from the scientific community due to the applications of colloidal semiconductors within two rapidly advancing research fronts heterogeneous photocatalysis and nanocrystalline particle technology. 

One of the most attractive features of colloidal semiconductor systems is the ability to control the mean particle size and size distribution by judicious choice of experimental conditions (such as reactant concentration, mixing regimen, reaction temperature, type of stabilizer, solvent composition, pH) during particle synthesis. Over the last decade and a half, innovative chemical [69], colloid chemical [69-72] and electrochemical [73-75] methods have been developed for the preparation of relatively monodispersed ultrasmall semiconductor particles. Such particles (typically <10 nm across [50, 59, 60]) are found to exhibit quantum effects when the particle radius becomes smaller than the Bohr radius of the first exciton state. Under this condition, the wave functions associated with photogenerated charge carriers within the particle (vide infra) are subject to extreme... 

A complete treatment of interfacial charge transfer in colloidal semiconductor systems with band gap excitation should consider the following factors ... 

Kinetic analysis of photoinduced interfacial charge transfer processes in colloidal semiconductor systems is frequently complex. Indeed, it is diffi-... 

For colloidal semiconductor systems, Albery et al. observed good agreement between the value of the radial dispersion obtained from dynamic light scattering and the value found from application of the above kinetic analysis to flash photolysis experiments [144], It should be remembered that this disperse kinetics model can only be applied to the decay of heterogeneous species under unimolecular or pseudo-first order conditions and that for colloidal semiconductors it may only be applied to dispersions whose particle radii conform to equation (37), i.e., a log normal distribution. However, other authors [145] have recently refined the model so that assumptions about the particle size distribution may be avoided in the kinetic data analysis. 

The kinetic analysis [103] of electron transfer in colloidal semiconductor systems is often complex. Apart from the energetics of the conduction band of the semiconductor and the redox potential of the acceptor, factors such as the surface charges of the colloids, adsorption of the substrates, participation of surface states, and competition with charge recombination influence the rate of charge transfer at the semiconductor interface [102], This fact is evident from the widely differing rates of experimentally observed charge transfer rates, with time scales ranging from picoseconds to milliseconds for different experimental conditions and various semiconductor systems. 

Fig. 5.17 CdS-ZnO coupled semiconductor system (a) interaction between two colloidal particles showing the principle of the charge injection process and (b) light absorption and electron transfer on an electrode surface leading to the generation of photocurrent. (Reproduced from [330])...

Hotchandani S, Kamat P (1992) Charge-transfer processes in coupled semiconductor systems. Photochemistry and photoelectrochemistry of the colloidal cadmium sulfide-zinc oxide system. J Phys Chem 96 6834—6839... 

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],... 

Heterogeneous semiconductor systems involve either suspensions or slurries of larger-sized semiconductor powders, or smaller colloids in solution. In principle, these semiconductor particles may act as tiny photoelectrolysis cells, similar to the photoelectrochemical systems discussed above. However, as many of the materials used for bulk electrodes are also described here in particulate form, both similar and new problems may arise, most notably irreproducibility in particle preparation, stability issues, and low C02 reduction rates. 

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.

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,... 

This review concentrates on John Albery s work in the field of colloidal semiconductor photoelectrochemistry. John s major contributions to this area, as in so many others, have been through his astounding facility for generating useful asymptotic solutions for highly complex kinetic models of electrochemical systems. So as to put John s work in colloidal photoelectrochemistry into context. Sections 9.1-9.3 of this chapter provide a review of the more salient kinetic models of semiconductor photocatalysis developed over the last 20 years or so. Section 9.4 then concentrates on the Alberian view and presents, for the first time, John s model of the chronoamperometric behaviour of colloidal CdS. 

In the case of colloidal semiconductors whose interfacial charge transfer kinetics are consistent with those assumed during the derivation of equation (9.28), the rate of charge transfer from the particles to the charge carrier acceptor in solution is proportional to the surface area of the particles. Albery et al. assume that, within the systems that they studied, the particle radii conform to a Gaussian distribution. Thus, in analogy to equations (9.30) and (9.31) ... 

Colloidal Semiconductors in Systems for the Sacrificial Photolysis of Water. 1. Preparation of a Pt/Ti02 Catalyst by Heterocoagulation and its Physical Characterization 254... 

An alternative way to achieve the photodissociation of water consists in the use of aqueous suspensions of powdered or colloidal semiconductors, in general loaded with noble-metal and/or noble-metal-oxide catalysts which act as short-circuited photoelectrolysis cells. Titanium dioxide was certainly (and is still being) the semiconductor most frequently employed in such systems. 

More recently, Fendler and coworkers have used surfactant vesicles as hosts for colloidal semiconductor particles [83]. For instance, dihexadecyl phosphate (DHP) and 2Ci8Br vesicles incorporating CdS, ZnS and mixed CdS-ZnS particles have been obtained and used for the realization of nanoscale photoelectric devices (see later). It is important to point out, however, that the presence of the closed vesicle does not appear, at least with these systems, to be of critical importance for the control of the size of the particles, as similar phenomena also occurs in Langmuir-Blodgett films [84], suggesting that a crucial role in nucleation is played mainly by the surface. 

Quite promising may be the study of photoelectrochemical properties of microheterogeneous semiconductor systems (porous electrodes, suspensions of different degrees of dispersion up to colloidal solutions), in which photosensitivity inherent in semiconductors is combined with high electrocatalytic activity typical of systems with a developed surface. 

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