Electron-hole pair formation

Figure 4.1 Schematic representation of the processes involved after absorption by a semiconductor particle of a photon of wavelength ofenergy equal to or higherthan g (a) electron-hole pair formation (b) oxidation...

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

The second spectrum (figure 3b) displays the spectral features of DPP+ radical cation and provides evidence of DPP spontaneous ionization DPB + HZSM-5 -> DPB + HZSM-5 " (eq. 2). The third spectrum (fig. 3c) exhibits a broad band at 425 nm and is assigned to electron-hole pair formation DPB + HZSM-5 " - DPB HZSM-5 + (eq. 3). 

Recombination of DPB+ radical cation can be summarized according to the reactions relating to either direct recombination (DPB+ HZSM-5 - DPB HZSM-5 (Eq. 4)) or to the capture of another electron of the framework by DPB+ (Eq. 3) and electron-hole pair formation as shown above. 

The photoreactivity of the involved catalyst depends on many experimental factors such as the intensity of the absorbed light, electron-hole pair formation and recombination rates, charge transfer rate to chemical species, diffusion rate, adsorption and desorption rates of reagents and products, pH of the solution, photocatalyst and reactant concentrations, and partial pressure of oxygen [19,302,307], Most of these factors are strongly affected by the nature and structure of the catalyst, which is dependent on the preparation method. The presence of the impurities may also affect the photoreactivity. The presence of chloride was found to reduce the rate of oxidation by scavenging of oxidizing radicals [151,308] ... 

Fig. 2-16. Electron state density distribution and electron-hole pair formation in the conduction and valence bands of intrinsic semiconductors Cf > Fermi level of intrinsic semiconductors.

Fig. 3-11. Energy for decomposing ionization of compound AB to form gaseous ions A(giD) and via electron-hole pair formation and via cation-anion vacancy pair formation r = reaction coordinate of decomposing ionization e, s semiconductor band gap . vmb) = cation-anion vacancy pair formation energy (Va- Vb-) Lab = decomposing ionization energy of compound AB.

The decomposing ionization will take place preferentially by way ofthe electron-hole pair formation, if the formation energy of the electron-hole pair, e, is smaller than the formation energy of the cation-emion vacancy pair, Hv(ab>, and vice versa. In general, compound semiconductors, in which the band gap is small (e,< Jfv(AB>), will prefer the formation of electron-hole pairs whereas, compound insulators such as sodium chloride, in which the band gap is great (e(>Hv(AB>), will prefer the formation of cation-anion vacancy pairs [Fumi-Tosi, 1964]. 

Effective dyes were those shown by Watanabe (103) and Calvin (104) to be strongly adsorbed to the pigment surface and to sensitize photoconductivity or photoredox reactions in the semiconductor materials. The initial interaction (eq. 46) is dye sensitized electron-hole pair formation in the semiconductor. 

Fignre 5.6 illnstrates the temporal evolution of the transient spectra of one of the sols examined, the 23 A TiOi specimen, at varions delay times after excitation with the 30 ps, 2.5 mJ laser pulse (Serpone et al, 1995b). Subsequent to electron-hole pair formation and charge-carrier separation, in competition with recombination, a fraction of these carriers are trapped at lattice sites and some migrate to the surface in about 0.05-10 ps, depending on particle size, where they also get trapped to give Ti " species for the electron and Ti" -0 -Ti" - OH for the hole (Lawless et al., 1991). 

Semiconductor electrodes whose band gap is relatively narrow receive photon energy and produce photoexcited electron-hole pairs in the space charge layer. The photoexcited electron-hole pair formation significantly increases the concentration of minority charge carriers (holes in the n-type), but influences little the concentration of majority carriers (electrons in the n-type). The photoexcited electrons and holes set their energy levels not at the electrode Fermi level, ef, but at what we call the quasi-Fermi levels, n p and p p, respectively. The quasi-Fermi level for majority carriers is close to the electrode Fermi level, F, but the quasi-Fermi level for minority carriers is far away from the electrode Fermi level. 

Scheme 17.1 Schematic representation of electron-hole pair formation in Ti02 nanoparticles and possible redox processes that may occur in air-saturated aqueous medium.

Fig. 1.1 Band structure of an n-type photoanode water splitting device, (a) Illustrating the various processes of photon irradiation, electron-hole pair formation, charge transport, and interfadal reactions, (b) Illustrating the energetic requirements associated with the minimum thermodynamic energy to split water, catalytic overpotentials for the HER and OER half-reactions, and photovoltage...

Fig. 2. Calculation (lines) and obsereved activation energies of carrier generation (bottom) and quantum yields of electron-hole pair formation (top). From Bounds et al. (19). Circles and squares taken from (6) and bars from L. E. Lyons and K. A. Milne,...

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