Photocatalysis electron-hole recombination

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. 

B) Electrochemically-assisted photocatalysis As discussed above, photocatalytic processes are electrochemical in nature. A clever enhancement approach involves the application of a judiciously selected potential bias to a semiconductor electrode. The potential promotes a better charge-separation, thus decreasing the electron-hole recombination and increasing the yield of the target processes. This approach is called electrochemically... 

Keywords Photocatalysis and catalysis Photocatalytic activity Band structure and excitation Energy conversion Langmuir-Hinshelwood mechanism Electron-hole recombination Quantum efficiency Physical property-activity correlation Synergetic effect. 

To overcome the intrinsic process limitations of electron/electron-hole recombination which lead to low quantum yields and PTEF values, photocatalysis can be developed under enhanced electron and hole-scavenging processes. This can be achieved in processes where the simultaneous conversion of organic and inorganic species takes place. 

The reaction rate of photocatalysis is commonly slow because of the phenomenon of electron-hole recombination. The limited activating UV spectrum in sunlight for Ti02 also explains why solar photocatalysis has a low photonic efficiency. A number of strategies have been developed to modify photocatalysts for visible-light response, such as using metal (Lin et al., 2010), metal oxide (Peng et al., 2011), and metal sulfide (Li et al., 2010) as modifiers. 

One of the major limitations in semiconductor photocatalysis is the relatively low value of the overall quantum efficiency mainly due to the high rate of recombination of photoinduced electron-hole pairs at or near the surface. Some success in enhancing the efficiency of photocatalysts... 

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]. 

Heterogeneous photocatalysis steps were reviewed by Cunningham and Hodnett, (1981), Jacoby etal., (1996) and Yue( 1993) among others. These steps include the mass transfer of substrate from the bulk of the fluid to the catalyst surface, the transport of the reactants within the catalyst particle and the adsorption of substrates on the active catalytic surface. Once the Ti02 is irradiated, photon energy is absorbed, followed by the generation of electron-hole pairs, the formation of radicals, the surface reaction, radical recombination and finally the desoiption and mass transfer of products from the particle surface into the bulk of the fluid. 

The recombination of electron/hole pairs can take place either between energy bands or on the surface. As a result the photocatalytic efficiency is reduced. To impede the recombination process, conducting materials such as noble metals can be incorporated into the semiconductor to facilitate the electron transfer and prolong the lifetime of the electron/hole separation process (10). Although, there has been considerable efforts in using photocatalysis for complete oxidation of organic compounds in air and water streams, incomplete or partial oxidation has been reported (//, J2). 

The diameter of the electrospun Ti02 nanofibers has also been found to play an important role on their photocatalytic activity. Pan and co-workers prepared pure anatase Ti02 nanoflbers with different diameters (92, 120, 205, 245 nm) via an electrospinning process and followed the calcination treatment only by adjusting the content of tetrabutyl titanate in the precursor solution [17]. The photocatalytic property studies showed that the activity increased with the increasing of the fiber diameter up to about 200 nm and then it decreased (Fig. 15.2). The photocatalysis mechanism might be due to dynanfics of the recombination of electron/hole on the surface sites. 

Fig. 11.7 Important processes in the surface photocatalysis processes photoexcitation of electron-hole pair, charge transfer processes, bulk and surface recombination processes, and electron- and charge-induced chemistry at surfaces (Reproduced with permission from Ref [27]. Copyright 1995 American Chemical Society)...

Fig. 11.29 A newly proposed photocatalysis model based on nonadiabatic chemical dynamical processes and ground state reactions. In this model, photoexcited electron-hole pairs are nonadiabatically recombined to convert the excited electronic state taiergy to the ground state energy, which drives the chemical reactions (m the ground state surface (Reprinted with permission from Ref. [192]. Copyright 2015 Dalian Institute of Chemical Physics, the Chinese Academy of Sciences)...

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