Photocatalysis positive hole

Many dyes are decolorized and ultimately mineralized by photocatalysis. In the degradation of azo dye,18 21) the degradation rate decreases in the order monoazo > diazo > triazo.1819) Three processes including oxidation and reduction are considered to occur simultaneously in the photocatalytic degradation of dye. These are illustrated in Fig. 9.9. Process 1 is the common photocatalytic degradation process of organic compounds. Process 2 is spectral sensitization as observed in a wet-type solar cell. In process 3 one moiety of dye molecule serves as the electron acceptor, suppressing recombination between electron and positive hole. 

Light with supraband gap energy absorbed by a semiconducting particle (e.g., Ti02) can excite an electron from its valence band to the conduction band. This process also creates a positive hole in the valence band, which is an oxidizer and may react with adjacent oxygenated species to produce OH radicals. At the same time, the excited electron is capable of directly or indirectly reducing species in contact with the semiconductor. Such phenomena are electrochemical in nature—they are under intense study for application in the environmental remediation arena where they are globally called photocatalysis. 

The principle of photocatalysis is often explained with an illustration like Fig. 2, a schematic representation of the electronic structures of semiconducting materials, a band model. An electron in an electron-filled valence band (VB) is excited by photoirradiation to a vacant conduction band (CB), which is separated by a forbidden band, a band gap, from the VB, leaving a positive hole in the VB (Section III.B). These electrons and positive holes drive reduction and oxidation, respectively, of compounds adsorbed on the surface of a photocatalyst. Such an interpretation accounts for the photocatalytic reactions of semiconducting and insulating materials absorbing photons by the bulk of materials. In the definition of photocatalysis given above, however, no such limitation based on the electronic structure of a photocatalyst is included. For example, isolated... 

A significant problem in studies on photocatalysis is the definition of positive hole. Positive hole is defined as a defect of an electron (i.e., a positive hole must be included in a substance, while an electron is a real substance). Therefore, not only h produced by photoinduced band-to-band transition in solid materials but also a hydroxyl radical, which is a one-electron deficient hydroxyl anion, can be a positive hole. If this definition is accepted, there should be no difference in the photocatalsrtic oxidation mechanisms between direct hole transfer and surface-adsorbed hydroxyl radical reaction, since it is well known that the surface of a metal oxide is covered with chemically or physically adsorbed water and a positive hole passing through this water layer into a solution may be a hydroxyl radical or its protonated or deprotonated species (Fig. 4). Actually, hydroxyl... 

Since an ordinary photocatalysis is induced by photoexcited electrons and positive holes, rate of photocatalytic reaction must depend on photoirradiation irradiance (light flux) and efficiencies of both photoabsorption and electron-positive hole utilization. The efficiency of electron-positive hole utilization is called... 

However, the rates of the photocatalytic chemical transformations are limited by the rates of electron hole recombination in the bulk of Ti02 or at the surface (Fig. 1). These latter rates depend particularly on structural defects and on foreign cations in substitutional and interstitial positions. They are not easily controlled and consequently limit the application fields of photocatalysis. 

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