Band gap excitation

A number of electronic and photochemical processes occur following band gap excitation of a semiconductor. Figure 5 illustrates a sequence of photochemical and photophysical events and the possible redox reactions which might occur at the surface of the SC particle in contact with a solution. Absorption of light energy greater than or equal to the band gap of the semiconductor results in a shift of electrons from the valence band (VB) to... 

The band-gap excitation of semiconductor electrodes brings two practical problems for photoelectrochemical solar energy conversion (1) Most of the useful semiconductors have relatively wide band gaps, hence they can be excited only by ultraviolet radiation, whose proportion in the solar spectrum is rather low. (2) the photogenerated minority charge carriers in these semiconductors possess a high oxidative or reductive power to cause a rapid photocorrosion. 

Fig. 96. Schematic illustration of a colloidal semiconductor. Band-gap excitation promotes electrons from the valence band (VB) to the conduction band (CB). In the absence of electron donors and/or acceptors of appropriate potential at the semiconductor surface or close to it, most of the charge-separated, conduction-band electrons (e CB) and valence-band holes (h+VB) non-pro-ductively recombine. Notice the band bending at the semiconductor interface [500]...

AOT-isooctane-H20 reversed micelles 50-A-diameter CdS particles generated in situ in reversed micelles from CdCl2 or Cd(N03)2 by H2S Reversed-micelle-entrapped CdS was fluorescence quenched by methylviologen band-gap excitation in the presence of Rh as catalyst and PhSH as sacrificial electron donor resulted in water photoreduction 611... 

Band-gap excitation of PS-BLM-incorporated CdS by a laser pulse generated an electron-hole pair which, in turn, resulted in the reduction of oxygen on one side of the BLM and in the oxidation of HS- on the other side. These processes produced a photovoltage across the membrane [651]. Three different... 

Figure 28.1 The electronic structure of a solid can be described in terms of a band model in which bonding electrons are primarily found in a low-energy valence band, while conduction is typically associated with antibonding or nonbonding high-energy orbitals known as the conduction band. In the case of a semiconductor (left), these two bands are separated by a quantum-mechanical forbidden zone, the band gap. Excitation of electrons from the valence band to the conduction band gives rise to the bulk optical and electronic properties of the semiconductor. In the case of a metal (right), the conduction band and valence band overlap, giving rise to a continuum of states.

The semiconductor-sensitized reaction is presumed to occur according to the sequence shown in Scheme 2. Band gap excitation creates an electron-hole pair at the semiconductor surface. 

In the absence of a photosensitizer, Ti02/Pt/Ru02 is reported to produce H, and 02 on UV photolysis in water, by direct band gap excitation, with a quantum yield of 0.3. 97... 

There are a number of advantages of using colloidal, semiconductors in artificial photosynthesis. They are relatively inexpensive. They have broad absorption spectra and high extinction coefficients at appropriate band gap energies. Nevertheless, they can be made optically transparent enough to allow direct flash photolytic investigations of electron transfers. They can be modified by derivatization or sensitizer adsorption. Importantly, electrons produced by band gap excitation can be used directly without relays for catalytic water reduction (Figure IB). 

The reaction can be followed by monitoring the characteristic absorption due to ZnO band-gap excitations, occurring at energies above 28,000 cm-1. The energy of the first excitonic transition depends on the nanocrystal size, and so provides a probe of nanocrystal growth. For a fixed size, the intensity of the transition provides a measure of the nanocrystal concentration (i.e., the yield of the chemical reaction), and this offers a measure of nucleation yields. 

Tb3+ The effect of the doping concentration on the optical spectra of Tb3+ in ZnO nanocrystals (5 nm) was investigated in details (Liu et al., 2001b). The PL intensity of Tb3+ centers increases with increasing Tb content at the expense of emission from defect states in the ZnO nanocrystals. The characteristic emission of Tb3+ at 544 nm is the strongest upon excitation of the ZnO host at 345 nm, which implies an efficient carrier relaxation from ZnO hosts to Tb3+ centers. For a 3-nm ZnO sample, the band-gap excitation is blue-shifted to 315 nm due to quantum size confinement. This significant ET from the ZnO nanociystal host to Tb3+ centers confirmed that Tb3+ ions can to some extent be effectively incorporated into ZnO nanocrystals. 

Yanes et al. (2004) observed a very interesting size selective spectroscopy in 0.4 mol% Eu3+ SnC>2 nanocrystals ( 4 nm) embedded in SiC>2 glass prepared by thermal treatment of sol-gel glasses. The mean size of SnC>2 nanocrystals is comparable to the bulk exciton Bohr radius (4.8 nm). Thus the band-gap excitation energy depends on the nanocrystal size. 

The structure of the BLM can be altered by introducing microcrystalline aggregates or films of inorganic metal sulfide semiconductors in the BLM [114]. The resulting film, which was about 1000 A thick and penetrated the BLM by about 17-18 A, was stable for days and produced stable photovoltages upon band gap excitation. Such semiconductor-loaded films could be polymerized via photochemical sensitization by band gap excitation of the semiconductor [115]. 

PRIMARY PROCESSES ON COLLOIDAL SEMICONDUCTORS 9.2.1 Light harvesting by semiconductor band gap excitation... 

Fig. 9.1. (a) Semiconductor particle charge transfer processes induced by direct band gap excitation. 

A complete treatment of interfacial charge transfer in colloidal semiconductor systems with band gap excitation should consider the following factors ... 

The nominal process involved in the photoexcitation of the Fe(III) oxides is band-gap excitation to produce transient valence band holes and conduction band electrons (Fig. 8) as follows ... 

Thermochemical splitting of water involves heating water to a high temperature and separating the hydrogen from the equilibrium mixture. Unfortunately the decomposition of water does not proceed until temperatures around 2500 K are reached. This and other thermal routes are discussed in Chapter 5. Solar thermal processes are handicapped by the Carnot efficiency limits. On the other hand, solar photonic processes are limited by fundamental considerations associated with band-gap excitation these have been reviewed in Refs.32 and 33. 

---