Semiconductor particles, photocatalysis

Dye sensitization of semiconductor surfaces is not considered here, nor are issues related to semiconductor particles, photocatalysis and photoelectrolysis per se. These companion topics may be found elsewhere in Volumes I, IV and V. The discussion is phenomenological and is designed to provide an intuitive grasp of the key issues rather than detailed derivations that would have been prohibitive in terms of space constraints in any case. Indeed, the available theoretical framework is only examined in terms of how and with what confidence the pertinent conclusions can be experimentally verified with semiconductor electrodes. 

Photocatalysis uses semiconductor materials as catalysts. The photoexcitation of semiconductor particles generates electron-hole pairs due to the adsorption of 390 run or UV light of low wavelength (for Ti02). If the exciting energy employed comes from solar radiation, the process is called solar photocatalysis [21],... 

The limiting factors that control photocatalysis efficiency are rapid recombination between photo-generated charge carriers, and the backward reaction leading to recombination of the formed molecular hydrogen and oxygen. To retard these processes efforts have typically focused on surface modification of the semiconductor particles using metals or metal oxides. 

Fig. 5.3 Processes of photocatalysis at semiconductor particles. The numbers correspond to those in the text, Path increased by decreasing the particle size are shown by increase. ...

Kinetic studies of photoreactions on semiconductor nanoparticles are important for both science and practice. Of scientific interest are the so-called quantum size effects, which are most pronounced on these particles shifting the edge of adsorption band, participation of hot electrons in the reactions and recombination, dependence of the quantum yield of luminescence and reactions on the excitation wavelength, etc. In one way or another all these phenomena affect the features of photocatalytic reactions. At present photocatalysis on semiconductors is widely used for practical purposes, mainly for the removal of organic contamination from water and air. The most efficient commercial semiconductor photocatalysts (mainly the TiC>2 photocatalysts) have primary particles of size 10-20 nm, i.e., they consist of nanoparticles. Results of studying the photoprocesses on semiconductor particles (even of different nature) are used to explain the regularities of photocatalytic processes. This indicates the practical significance of these processes. 

Photocatalysis, i.e., using semiconductor particles under band gap irradiation as little micro reactors for the simultaneous reduction and oxidation of different redox systems, has been intensively studied during the last 25 years since the pioneering work of Carey et al [1]. The main focus of these studies seems to be the investigation of the principal applicability of photocatalytic systems for the efficient treatment of water and air streams polluted with toxic substances. Several review articles on this topic have recently been published [2]. In some cases, pilot-scale or even commercially available reactors have already been constructed, especially when titanium dioxide is used as the photocatalyst [3]. 

However, due to the inherent complexity of this minute photoelectro-chemical system, details of the underlying reaction mechanisms of photocatalysis are even today still far from being understood. In contrast to an ordinary photoelectrochemical cell which employs an external bias voltage to deliberately separate oxidation and reduction processes in different compartments of the reactor, in photocatalysis both processes occur on the surface of the same semiconductor particle, usually only separated by a distance of a few angstroms. Moreover, as is evident from basic principles, the reaction rate of the overall process will be limited by the... 

Fig. 7.1. Schematic presentation of the processes occurring in photocatalysis upon bandgap irradiation of a semiconductor particle...

Bavykin, Dmitry V. is a Ph.D. researcher in the Laboratory of photocatalysis on semiconductors at the Boreskov Institute of Catalysis, Novosibirsk, Russia. The title of his PhD thesis (1998) Luminescent and photocatalytic properties of CdS nanocolloids . Area of his interests is the photophysical-photochemical properties of nanosized sulfide semiconductors, including synthesis of particles with definite size and surface properties, their characterisation the study of the photoexcited states dynamics, relaxation in quantum dots by the luminescence and flash photolysis measurements studies of the interfacial charge transfer from colloidal semiconductor particles by the steady state photolysis, luminescence quenching method. 

Photocatalytic methods involve illumination of a large band gap semiconductor particle such as Ti02 either dispersed as a slurry in the contaminated aqueous solutions or as immobilized films. While a substantial body of literature exists on photocatalysis [29-40], barriers to successful commercialization still prevail. The major problems with this technique include deactivation of the photocatalyst surface, and recovery of the photocatalyst in slurry systems. 

Figure 7.14 Mechanism of photocatalysis at a semiconductor particle. (Adapted from Kisch and Macyk [82])...

Heterogeneous photocatalysis on semiconductor particles has been shown to be an effective means of removing toxic organic pollutants as well as toxic metal ions from water. In the first part of this chapter, systematic kinetic... 

Since photoexcitation induces significant enhancement of the reactivity of electron transfer, photochemical reactions via photoinduced electron transfer have been explored in homogeneous systems [43 52], On the other hand, the term photocatalysis has usually been used in heterogeneous systems involving photoinduced electron transfer across the gas-solid or liquid-solid interface [53-60], Photocatalysis has been extensively studied using a semiconductor particle as a photocatalyst [53-60], Photocatalysis is initiated by the absorption of a band gap photon... 

Stimulated by the early work of Bard et al. on the Ti02-catalyzed photo-Kolbe reaction [36], many papers appeared in subsequent years dealing with photocatalytic organic reactions [115] in the presence of colloidal or suspended semiconductor particles. They include cis-trans isomerizations [68, 93, 116-119], valence isomer-izations [120, 121], substitution and cycloaddition reactions [73, 80, 122-125], oxidations [126, 127], and reductions [128-130]. Characteristic of all these reactions is that in almost all cases well known compounds were formed, which were not isolated but only characterized by spectroscopic methods. The nature of the products can be rationalized within the mechanistic scheme of semiconductor photocatalysis type A, which means that at least one reduced and one oxidized compound are... 

Semiconductor particles have been used to induce efficient photoreactions of organic substrates for synthetic applications [4], Recently the study was extended to semiconductor nanoclusters [148-152]. The ZnS nanoclusters (2-5 nm) and their aggregates were found to be effective catalysts for the photoreduction of aliphatic ketones to alcohols [150], The coexistence of both S2 and SO3 is required for effective photocatalysis to occur. The observed overall reactions, using 2-butanone as an example, can be summarized as [150]... 

Homogeneous doping of semiconductor particles with a small amount of metal ions such as Fe " and V " prolongs the electron-electron-hole separation and hence increases the photocatalytic efficiency [46,47]. However, doping of Ti02 with metal ions such as Cr + and Sb + creates electron acceptor and donor centers that accelerate the charge recombination—an undesirable result for photocatalysis [48-50]. 

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