Electron-hole separation

Irradiation of a semiconductor with light of quantum energy greater than the band gap can lead to electron-hole separation. This can affect adsorption and lead to photocatalyzed or photoassisted reactions [187]. See Section XVIII-9F for some specifics. 

Experimental Values of Charge-Generation Efficiencies. In this section the charge-generation efficiencies of many polymeric photoconductors are compared (Table 3). When the experimental data has been fitted to the Onsager model, the initial electron—hole separation distance,... 

All of these uses are based on the behavior of titanium dioxide as a semiconductor. Photons having energies greater than v 3.2 eV (wavelengths shorter than 400 nm) produce electron/hole separation and initiate the photoreactions. Electron spin resonance (esr) studies have demonstrated electron capture by adsorbed oxygen to produce the superoxide radical ion (Scheme 1) (11). Superoxide and the positive hole are key factors in photoreactions involving titanium dioxide reported here are the results of attempts to alter the course of these photoreactions by use of metal ions and to understand better the mechanisms of these photoreactions. 

Semiconductor - Metal Junctions Besides the semiconductor-liquid interface, electron-hole separation can be attained also when the couple is generated in the space charge layer of a homo/heterojunction or semiconductor-metal junction. The metal can also act as electrocatalyst (e.g., for reduction of 02, H+ or C02). The development of the proper structure, including arrays of multiple junctions in series to enhance photovoltages and efficiently harvest radiation [53] and/ or the inclusion of suitable electrocatalysts, is crucial. 

From the framework depicted, it emerges that photocatalytic activity seems strictly related to the dipole moment generated by a distorted crystal structure, namely electron-hole separation upon photoexcitation is promoted by a local electric field due to a dipole moment and, in turn, this promotes vectorial movement of electron and holes. 

As seen in reaction (6.5.3) photogenerated holes are consumed, making electron-hole separation more effective as needed for efficient water splitting. The evolution of CO2 and O2 from reaction (6.5.6) can promote desorption of oxygen from the photocatalyst surface, inhibiting the formation of H2O through the backward reaction of H2 and O2. The desorbed CO2 dissolves in aqueous suspension, and is then converted to HCOs to complete a cycle. The mechanism is still not fully understood, with the addition of the same amount of different carbonates, see Table 6.2, showing very different results [99]. Moreover, the amount of metal deposited in the host semiconductor is also a critical factor that determines the catalytic efficiency, see Fig. 6.7. 

Various pairs of inorganic ions such as lOsVr, Fe /Fe, and Ce /Ce have been used as redox mediators to facilitate electron-hole separation in metal loaded oxide semiconductor photocatalysts [105-107], Two different photocatalysts, Pt-Ti02 (anatase) and Ti02 (rutile), suspended in an aqueous solution of Nal were employed to produce H2 and O2 under, respectively, the mediation of 1 (electron donor) and IOs (electron acceptor) [105]. The following steps are involved in a one-cell reaction in the presence of UV light. 

Incorporation of n-Si in the mixed CdS-ZnS system results in electron flow from the n-Si to the CdS conduction band, resulting in greater utilization of solar radiation as well as superior electron-hole separation [162,163]. Under direct sunlight (25 mW/cm ), a suspension of n-Si/CdS-ZnS yields 15 mL/h/g of hydrogen from an aqueous sulfite/sulfide solution [162]. Hydrogen evolution of 35 mL/h/g has been recorded for CdS-ZnS (2 1 weight ratio) prepared by coprecipitation and then loaded with n-Si (1.4 - 1.6%) [163]. 

AeF (n) = change in Fermi energy due to steady state electron-hole separation in the diffusion potential zone of a p-n junction. 

Metals, such as platinum, are usually introduced to improve the electron-hole separation efficiency. In order to analyze the energy structure of the metal-loaded particulate semiconductor, we solved the two-dimensional Poisson-Boltzmann equation.3) When the metal is deposited to the semiconductor by, for example, evaporation, a Schottky barrier is usually formed.45 For the Schottky type contact, the barrier height increases with an increase of the work function of the metal,4 which should decrease the photocatalytic activity. However, higher activity was actually observed for the metal with a higher work function.55 This results from the fact that ohmic contact with deposited metal particles is established in photocatalysts when the deposited semiconductor is treated by heat65 or metal is deposited by the photocatalytic reaction.75 Therefore, in the numerical computation we assumed ohmic contact at the energy level junction of the metal and semiconductor. 

When the size of the particle decreases, the electron-hole separation is expected to be inefficient. As has been postulated in the 2D ladder reaction model stated above, the recombination rate is reciprocally proportional to the volume of the particle. That is, the recombination rate is proportional to the probability at which an electron encounters a hole. Since the surface electron transfer reaction competes with the recombination reaction, the transfer efficiency decreases with the decrease of the size of the particle, if the rate of the surface electron transfer does not change significant . ... 

Light absorption, by markedly affecting the electronic properties of molecules and metal complexes, may induce intra- or intermolecular electron transfer processes leading to electron-hole separation. 

Figure 2. Electron-hole separation on a metallized (M) semiconductor (SC) powder.

Fig. 12.21. The effect of incident light on an n-type semiconductor and on electron transfer. Electron-hole separation is promoted only in the space-charge region. The energy of the redox couple, Eredox, determines if there is oxidation or...

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