Electron-hole separation, metallized

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. 

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. 

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. 

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.

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]. 

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). 

Through combinations of the equihbrium constant expressions and electroneutrality above, the concentration of the separate point defects and electronic defects can be evaluated. In an oxide Mi.yO this leads to relationships similar to that for oxygen vacancies with the exception that the concentrations of electron holes and metal vacancies always increase with increasing oxygen pressure. The concentration of electron holes is, for instance, determined by the expreession... 

The reaction rates for these reactions linearly increased with increasing work function of the metal cocatalyst. The metal nanoparticles produce an electric field gradient, causing an efficient electron-hole separation. Some other noble metals, such as Au [258,327], Rh, Pd [328], Ag, and Ru [329], have also been reported as efficient cocatalysts. Noble metals, including Ft, Au, Pd, and Rh have been reported to be very effective for enhancement of Ti02 and photocatalysis [241,319-322]. As the Fermi levels of these noble metals are lower... 

The properties of the band gap in semiconductors often control the applicability of these materials in practical applications. To give just one example, Si is of great importance as a material for solar cells. The basic phenomenon that allows Si to be used in this way is that a photon can excite an electron in Si from the valence band into the conduction band. The unoccupied state created in the valence band is known as a hole, so this process has created an electron-hole pair. If the electron and hole can be physically separated, then they can create net electrical current. If, on the other hand, the electron and hole recombine before they are separated, no current will flow. One effect that can increase this recombination rate is the presence of metal impurities within a Si solar cell. This effect is illustrated in Fig. 8.4, which compares the DOS of bulk Si with the DOS of a large supercell of Si containing a single Au atom impurity. In the latter supercell, one Si atom in the pure material was replaced with a Au atom,... 

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