Electron-hole formation

Fick s second law states the conservation of the diffusing species i no i is produced (or annihilated) in the diffusion zone by chemical reaction. If, however, production (annihilation) occurs, we have to add a (local) reaction term r, to the generalized version of Fick s second law c, = —Vjj + fj. In Section 1.3.1, we introduced the kinetics of point defect production if regular SE s are thermally activated to become irregular SE s (i.e., point defects). These concepts and rate equations can immediately be used to formulate electron-hole formation and annihilation... 

The photocatalytic reaction proceeds via a series of chemical events, following the initiation step of pair electron-hole formation. This leads to the utilization of both the electron-hole /i+ for oxidation processes and eventually to the capture of the e electron for reduction processes, as well as the potential formation of super oxides anions and hydrogen peroxide from oxygen. 

Figure 1. (A) Schematic representation of electron-hole formation and electron transfer reaction at the semiconductor particle. (B) Thermodynamic constraints for electron exchange at illuminated semiconductor-electrolyte interfaces (A=electron acceptor, D=electron donor).

The overall charge separation process across a semiconductor liquid junction involves various contributions to the mechanism, details of which are still lacking. These include the light induced electron-hole formation, their various recombination mechanisms and their transport across the interface to react with the electrolyte. Time resolved techniques, that can now reach the femtoseconds time scale, should be a powerful tool to elucidate many mechanistic and kinetic aspects of these processes and provide the best interface with theory. Care should be exercised in... 

Photovoltaic Devices. For many inorganic semiconductors, absorption of light can be used to create free electrons and holes. In an organic semiconducting soHd, however, absorption of a photon leads to the formation of a bound electron—hole pair. Separation of this pair in an electric field can... 

Current research aims at high efficiency PHB materials with both the high speed recording and high recording density that are required for future memory appHcations. To achieve this aim, donor—acceptor electron transfer (DA-ET) as the hole formation reaction is adopted (177). Novel PHB materials have been developed in which spectral holes can be burnt on sub- or nanosecond time scales in some D-A combinations (178). The type of hole formation can be controlled and changed between the one-photon type and the photon-gated two-photon type (179). 

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...

Platinum-loaded Ti02 systems can be considered as a short-circuited photo-electrochemical cell where the Ti02 semiconductor electrode and metal Pt counterelectrode are brought into contact [159]. Light irradiation can induce electron-hole (e -h +) pair formation and surface oxidation and also reduction reactions on each Pt/Ti02 particle (Figure 4.11). These powder-based systems lack the advantage of... 

The first step consists in the absorption of a quantum hv of short wavelength and the formation of an exciton (electron/hole pair) ... 

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. 

While most of the research in metastable defect formation has focussed on light-induced defects, there has recently been growing interest in thermally generated defects. Smith and Wagner (1985 Smith et al., 1986) extended the proposed Staebler-Wronski mechanism of electron-hole recombination via band tail states, resulting in the formation of dangling... 

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] ... 

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