Doping electron-hole recombination

As previously mentioned, a key point in the optimization of the catalysts photodegradation of phenol and its derivates, is the minimization of the electron-hole recombination and the intimate connection of this process with the anion vacancies present in the size-limited, nanometric oxide particles. Minimization of the overall amount of oxide defects has a significant impact on the reaction rate. Traditional methods for improving electron-hole charge separation beyond what can be obtained with bare titania, involves doping mainly with although surface... 

It was demonstrated that it is possible, for example, loading the catalyst surface with noble metals such as Au, Pt, Pd, Ni, Ag [6, 8-10] reducing the possibility of electron-hole recombination or doping the catalyst with metal ions such as Fe3+, Co2+, Cu2+, Al3+ [11-14] that could expand its photoresponse into the visible region. 

Doping. Studies (52,53) have shown that if substitutional cationic doping at low levels is homogeneous, which can be achieved by the use of a flame reactor (see Sec. II.B.2, Particle Size ), it has a detrimental effect on the photocatalytic activity under UV irradiation. Furthermore, no activity is observed under irradiation in the visible spectral range in spite of an absorption by these samples. These observations regarding various reactions in different media have been attributed to electron-hole recombination at the site of the foreign cations (52,54). 

The mechanisms of the photostimulated luminescence of Eu2+-doped fluorohalides have been extensively studied. In BaBrF Eu an excess of fluorine favours the formation of F centres in bromine sites. This results in a red shift of the absorption induced by X-rays which increases the luminescence yield for stimulation by a He—Ne laser at 633 nm [69], Various interpretations have been proposed both about the nature of the hole-trapping centre and the electron-hole recombination mechanism [70], It was initially assumed that holes are trapped by Eu2+, leading to the formation of Eu,+ [71]. However after long X-ray irradiation no change in the EPR signal of Eu2+ was observed and the luminescence of Eu3+... 

Harima et al. (1989) measured the photogeneration efficiencies of 5,10,15,20-tetraphenylporphyrin (TPP) and its Zn complex (ZnTPP) doped with tetracyano-quinodimethane (TCNQ), o-chloranil (Chi), phenothiazine (Pz), and I. TCNQ, Chi, and I are electron acceptors while Pz is a donor. The porphyrins were selected on the basis of differences in exciton diffusion lengths (Tanimura et al., 1980 Yamashita et al., 1987) and oxidation potentials (Felton, 1978). The photogeneration of I doped ZnTPP was described by a direct ionization process via a singlet state of ZnTPP and ascribed to a reduction of the electron-hole recombination rate in the depletion layer. For TPP doped with Chi, Pz, or I, the results were explained by exciplex dissociation (Loutfy and Menzel, 1980). For... 

Figure 11 Photographic latent image formation in undoped and formate-doped and gold-sulfide (S) sensitized AgBr crystals. Top undoped crystal with electron-hole recombination. Centre formate doped crystal and hole scavenging step by formate (HCOp). Bottom formate-doped crystal and delayed reduction step of additional silver ions by carboxyl radicals CO- [16],...

Fig. 2 Example of an optimized OLED structure. (1) (2) cathode, Ag and LiE (3) electron transporting layer Alq3 (=Alqol3). (4) hole blocking layer, BCP. (5) electron-hole recombination zone/emitter layer, PVK doped, for example, with Ir(ppy)3. (6) and (7) anode, PEDOT doped with PSS (6) for improvement of hole injection and substrate smoothness and ITO (7) (=indium tin oxide). (8) glass support. For further details and explanations see text...

Doping titanium dioxide with metal ions that have been substitutionally positioned in the place of titanium within the lattice is one approach and has been carried out extensively using metals such as iron, molybdenum and copper. With subsurface doping, movement of electron and hole pairs to the surface (where they can react with substrates) is difficult and the metal ions can also act as recombination centres. An optimum doping concentration, above which photocatalytic activity decreases due to promotion of such electron-hole recombination, exists. 

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