Photogeneration of holes

Figure 7.8 Photogeneration of holes at the depletion layer of an n-type semiconductor.

The first term describes the photogeneration of holes for illumination via the electrolyte, the second term represents reaction of holes with the reduced species and the third term describes irreversible trapping of holes by a distribution of trap states. The trapped holes may be consumed in the oxidation of the reducing agent or by recombination with electrons (see section 5.2). 

Figure 1. A schematic diagram (a) and a partial equivalent circuit (b) are given for the LAPS. The components 4, Ci, Cdf Re, Vref and Vchem, respectively represent the applied bias potential, the insulator and depletion layer capacitances, the electrolyte resistance, the potential across the reference electrode, and a chemically sensitive surface potential. Ip represents the photogeneration of hole-electron pairs, and I the measured alternating photocurrent. Solution potential is maintained by a potentiostat using a Pt controlling electrode (CTL) and Ag/AgCl reference electrode (REF). The potential is defined as the potential from the output of the reference electrode to ground.

Anodic polarization. Photogeneration of holes required for porous silicon from lightly doped n-type silicon Holes from substrate electrons from oxidation of silicon (Chazalviel and Ozanam1992) EL triggered by injection of holes into luminescent nanocrystals blue shift of EL during oxidation. EL limited in time because Si nanocrystals are irreversibly oxidized (HaUmaoui et al. 1991 Bsiesy et al. 1991 Hory et al. 1995 Cantin et al. 1996 Billat 1996)... 

Figure 4. Schematic illustration of the photogeneration of holes and electrons upon excitation of a semiconductor particle and the subsequent redox processes in (a) TiQz and (b) TiOj/Pt.

Excited-state wavefunction analyses arc carried out in the framework of the Intermediate Neglect of Differential Overlap/Single Configuration Interaction (INDO/ SCI) technique to characterize the properties of the photogenerated electron-hole pairs. The SCI wavefunction writes ... 

In conclusion, nanorods are a potentially interesting material, but present results still do not allow understanding of whether the nanostructure leads to an improvement of the intrinsic photocatalytic behaviour, or whether other factors (accessible surface area, enhanced adsorption, etc) are responsible for the observed differences. In ZnO nanorods have been shown quite recently by surface photovoltage spectroscopy that the built-in electrical field is the main driving force for the separation of the photogenerated electron-hole pairs.191 This indicates that the nano-order influences the photophysical surface processes after photogeneration of the electron-hole pairs. A similar effect could be expected for Titania nanorods. However, present data do not support this suggestion, mainly due to the absence of adequate photo-physical and -chemical characterization of the materials and surface processes. 

In the absence of any adsorbed species, the photogenerated electron-hole pair in TiCh can react with the available surface species that is, Ti(IV) and bridging O2-. 

Illumination is a relevant parameter in the electrochemistry of silicon because photogenerated carriers may initiate or contribute to the charge exchange at the electrolyte-silicon interface. If an electrode is illuminated, photogenerated electron-hole pairs are generated corresponding to the number of absorbed photons. This number depends on spectral distribution, total illumination intensity and losses due to optical reflection and transmission. The number of electron-hole... 

The potential, E, for the onset of the photoexdted reaction relative to the equilibrium electrode potential E of the same reaction can also be derived in a kinetics-based approach [Memming, 1987]. Here, we consider the transfer of anodic holes (minority charge carriers) at an n-type semiconductor electrode at which the hole transfer is in quasi-equilibrium then, the anodic reaction rate is controlled by the photogeneration and transport of holes in the n-type semiconductor electrode. The current of hole transport, has been given by Eqn. 8-71 as a function of polarization ( - ,) as shown in Eqn. 10-20 ... 

Pig. 10-19. (a) Capture of photogenerated holes in surface states to form siuface ions and (b) anodic dissolution of surface ions to form hydrated ions on an n-type semiconductor electrode Oj = rate of hole capture in surface states oqx = rate of anodic dissolution of surface ions Cn = surface state level S, = surface atom of semiconductor electrode h(vs) = hole in the valence band h(n> = hole captured in smface states h(soH-) = hole in dissolved ions. 

The holes injected by a cathodic redox reaction (Eqn. 10-47) diffuse toward the electrode interior and recombine with electrons of the m ority charge carriers in the same way as photogenerated holes, thereby producing a cathodic current inc, which is equivalent to the rate of recombination of holes. The cathodic current i actually observed is the sum of the current of recombination inc and the limiting diffusion current of holes ip.um as shown in Eqn. 10-48 ... 

It might be thought that photogeneration of electrons and holes in a semiconductor with normally a small population of one or other (or both) charge carrier would result in both hydrogen and oxygen production at the interface, through the reactions ... 

In this type of cell both electrodes are immersed in the same constant pH solution. An illustrative cell is [27,28] n-SrTiOs photoanode 9.5-10 M NaOH electrolyte Pt cathode. The underlying principle of this cell is production of an internal electric field at the semiconductor-electrolyte interface sufficient to efficiently separate the photogenerated electron-hole pairs. Subsequently holes and electrons are readily available for water oxidation and reduction, respectively, at the anode and cathode. The anode and cathode are commonly physically separated [31-34], but can be combined into a monolithic structure called a photochemical diode [35]. 

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