Holes valence-band

The primary photochemical act, subsequent to near-uv light (wavelengths <400 nm) absorption by Ti02 particles, is generation of electron—hole pairs where the separation (eq. 3) into conduction band electrons (e g ) and valence band holes (/lyB ) faciUtated by the electric field gradient in the space charge region. Chemically, the hole associated with valence band levels is constrained at... 

Depending on the nature of the electrode and reaction, the carriers involved in an electrochemical reaction at a semiconductor electrode can be electrons from the conduction band (in the following to be called simply electrons), electrons from the valence band (holes), or both. The concentration of the minority carriers in semiconductors (electrons in p-type, and holes in n-type semiconductors) is always much... 

Redudion reactions are generally less often used than photocatalytically assisted oxidations, mainly because the reduction power of a valence band electron is lower than the oxidation ability of a valence band hole. However, even in this case they can contribute to replacing dangerous reductants such as CO or hydrides with safer procedures. 

In these photocatalytic oxidation processes the organic compound is either directly oxidized by the valence band holes ... 

Electron donors, D, adsorbed on the particle surface react with valence-band holes... 

It is also assumed (Hoffmann 1990) that the adsorbed sulfite is oxidized by the valence band holes, h+b, that are formed through absorption of light with photon energies exceeding the band-gap energy (ca. 2.2 eV) of an iron(III)(hydr)oxide, e.g., hematite (a-Fe203). This interfacial electron transfer reaction results in formation of the SO radical anion which reacts with another radical to form S20 , one of the end product, if the reaction is carried out under nitrogen. 

In covalent semiconductors of single element S such as silicon, the covalent bonding electron is in the valence band and the valence band hole participates in the ionization of surface atoms as shown in Eqn. 3-13 and in Fig. 3-7 ... 

Figures 8-16 and 8-17 show the state density ZXe) and the exchange reaction current io( ) as functions of electron energy level in two different cases of the transfer reaction of redox electrons in equilibrium. In one case in which the Fermi level of redox electrons cnxEDax) is close to the conduction band edge (Fig. 8-16), the conduction band mechanism predominates over the valence band mechanism in reaction equilibrium because the Fermi level of electrode ensa (= nREDOK)) at the interface, which is also dose to the conduction band edge, generates a higher concentration of interfadal electrons in the conduction band than interfadal holes in the valence band. In the other case in which the Fermi level of redox electrons is dose to the valence band edge (Fig. 8-17), the valence band mechanism predominates over the conduction band mechanism because the valence band holes cue much more concentrated than the conduction band electrons at the electrode interface.

Similarly, the partial reaction currents, ip(ii) and ip(T ), carried by the valence band holes in Eqn. 8-54 may also be obtained. 

PL spectra of CdS deposited from two different acidic baths have been reported. From an acid thioacetamide bath, a broad band centered around ca. 1.5 eV was obtained [8]. The most likely cause for this luminescence was suggested to be valence band hole-S vacancy recombination. Films deposited under illumination from a thiosulphate solution exhibited a broad band from ca. 1.46-2.0 eV (peak at ca. 1.66 eV) [26]. 

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]...

Specific features of corrosion processes at semiconductors (as against to metals) are caused by the fact that charge carriers of both signs, namely conduction band electrons and valence band holes, take part in charge exchange between a solid and a solution. Therefore, the condition of Eq. (43) is insufficient, so account should be made of charge balance for each type of the carriers because equilibrium between the bands, which is established via generation-recombination processes, may not be reached. 

Under the hypotheses of constant illumination intensity, fast reaction of tlie electron scavenger with photogenerated electrons, and steady-state conditions applied to ecB nd hvg, a functional form like Eq. (2) was obtained without invoking adsorption [34], The rate expression was given as in the EH model by reaction of surface-active species with the substrate, in which A LH= h, and where h is the surface concentration of any oxidative active species. No assumptions were made on the steady-state concentrations of conduction-band electrons, valence-band holes, and other transient species. The rate is given by... 

Spectroscopic evidence for the transient formation of the trans-stilbene radical cation could be obtained when colloidal TiOj suspended in an acetonitrile solution containing trans-stilbene (a species which should also be exothermically oxidized by a TiO valence band hole) was excited with a laser pulse The observed transient was identical in spectroscopic features and in lifetime with an authentic sample of the stilbene cation radical generated in the same medium via pulse radiolytic techniques. That the surface influences the subsequent chemistry of this species can be seen in the distribution of products observed under steady state illumination, Eq. (4) 2 . ... 

At the n-type interface, the electric field generated causes photogenerated conduction band electrons to move into the bulk of the semiconductor, to the back metal contact, and into the external circuit. The valence band holes access the semiconductor interface due to the influence of the interfacial electric field (Fig. 28.2). Thus, redox species can be oxidized by the excited n-type semiconductor. These materials act as photoanodes. On the other hand, the electric field in a p-type material is reversed in potential gradient therefore, excited electrons move to the semiconductor surface, while holes move through the semiconductor to the external circuit (Fig. 28.2). These materials are photocathodes. The presence of an electric field at the semiconductor-electrolyte interface is usually depicted by a bending of the band edges as shown in Figure 28.2. Elec-... 

Although several single-crystal, wide-band gap semiconductors provide electrochemical and optical responses close to those expected from the ideal semiconductor-electrolyte model, most semiconducting electrodes do not behave in this manner. The principal and by far overriding deviation from the behavior described in the previous section is photodecomposition of the electrode. This occurs when the semiconductor thermodynamics are such that thermal or photogenerated valence band holes are sufficiently oxidizing to oxidize the semiconductor lattice [8,9]. In this case, kinetics routinely favor semiconductor oxidation over the oxidation of dissolved redox species. For example, irradiation of n-CdX (X = S, Se, or Te) in an aqueous electrolyte gives rise exclusively to semiconductor decomposition products as indicated by... 

Photocatalytic reactions at the semiconductor surface can be described by the following six steps as shown in Fig. 5.3. (D Absorption of a unit of light associated with the formation of a conduction band electron and a valence band hole in the semiconductor. (2) Transfer of an electron and a hole to the surface. (D Recombination of electron-hole pairs during the reaction processes. Stabilization of an electron and a hole at the surface to form a trapped electron and a trapped hole, respectively. (D Reduction and oxidation of molecules at the surface. (6) Exchange of a product at the surface with a reactant at a medium. Among these reaction steps, the absorption of light in the bulk (step CD) and... 

In the case of a metal-semiconductor junction the semiconductor surface is closely coupled to the metal. As a result electrons in the conduction band at the surface see a high density of empty metal states into which they can cross the interface isoenergetically. Similarly, valence band holes see a high density of filled electron states in the metal. As a result, electron transfer is usually treated as direct transfer in a thermionic process and surface states a re-rjs legated to a minor role in surface generation and recombination effects. -1—... 

At low light flux, the semiconductor sensitization is constrained to one electron routes, since the valence band hole is annihilated by a single electron transfer. Presumably after decarboxylation the resulting alkyl radical can be reduced to the observed monodecarboxylate more rapidly than it can transfer a second electron to form the alkene. In a conventional electrochemical cell, in contrast, the initially formed radical is held at an electrode poised at the potential of the first oxidation so that two-electron products cannot be avoided and alkene is isolated in fair chemical yield. Other contrasting reactivity can be expected for systems in which the usual electrochemistry follows multiple electron paths. 

Other methods used to obtain information on the photogeneration, transport, trapping, and recombination of conduction band electrons and valence band holes include... 

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