Fermi level, evolution

Fig. 4.2 Scheme of the Fermi level evolution in the frame of the rigid-band model (RBM). Two situations are considered case I represents the electron-gas RBM, case II shows the screened-impurity RBM. (B) and (A) mean before and after lithium insertion, respectively. In both cases, the cell voltage Foe >s the same... 

The photoelectrolysis of H2O can be performed in cells being very similar to those applied for the production of electricity. They differ only insofar as no additional redox couple is used in a photoelectrolysis cell. The energy scheme of corresponding systems, semiconductor/liquid/Pt, is illustrated in Fig. 9, the upper scheme for an n-type, the lower for a p-type electrode. In the case of an n-type electrode the hole created by light excitation must react with H2O resulting in 02-formation whereas at the counter electrode H2 is produced. The electrolyte can be described by two redox potentials, E°(H20/H2) and E (H20/02) which differ by 1.23 eV. At equilibrium (left side of Fig. 9) the electrochemical potential (Fermi level) is constant in the whole system and it occurs in the electrolyte somewhere between the two standard energies E°(H20/H2) and E°(H20/02). The exact position depends on the relative concentrations of H2 and O2. Illuminating the n-type electrode the electrons are driven toward the bulk of the semiconductor and reach the counter electrode via the external circuit at which they are consumed for Hj-evolution whereas the holes are dir tly... 

In connection with this problem it should be mentioned that 02-formation was found at CdS electrodes coated with polypyrrole and RUO2 under anodic polarization whereby the anodic decomposition could be considerably reduced. Under open circuit conditions only H2-evolution was observed, whereas O2 could obviously not be detected. This result is not in contradiction to the first experiment because the Fermi level can pass the electrochemical potential of H2O/O2 under bias. Very recently it was reported on photocleavage of H2O at catalyst loaded CdS-particels in the... 

Fig. 10-28. Polarization curves for cell reactions of photoelectrolytic decomposition of water at a photoezcited n-type anode and at a metal cathode solid curve M = cathodic polarization curve of hydrogen evolution at metal cathode solid curve n-SC = anodic polarization curve of oxygen evolution at photoezcited n-type anode (Fermi level versus current curve) dashed curve p-SC = quasi-Fermi level of interfadal holes as a ftmction of anodic reaction current at photoezcited n-type anode (anodic polarization curve r re-sented by interfacial hole level) = electrode potential of two operating electrodes in a photoelectrolytic cell p. sc = inverse overvoltage of generation and transport ofphotoezcited holes in an n-type anode.

Figure 10-30(c) applies to the photoexcited cell, where oxj en evolution proceeds via the anodic transfer of holes at the n-type anode and hydrogen evolution proceeds via the cathodic transfer of electrons at the p-type cathode. In order for the photoelectrolytic decomposition of water to proceed in such a cell, the edge level of the valence band sCy of n-type anode needs to be lower than the Fermi level tr(02ai20) of oxygen redox reaction and the edge level of the conduction band p c of p-type cathode needs to be higher than the Fermi level of... 

Photoelectron spectroscopy is a highly surface sensitive technique because of the inelastic mean free path of the photoelectrons Ae, which depends on the electron kinetic energy Ekin and has typical values of 0.2-3nm [31,37,38]. Determination of Schottky barrier heights b, or valence band discontinuities AEyB, can be performed by following the evolution of the position of the valence band maxima with respect to the Fermi level of substrate and overlayer with increasing thickness of the overlayer. For layer-by-layer growth the attenuation of the substrate intensities is given by the inelastic... 

Fig. 4.24. Evolution of CdS and ZnO valence band maxima positions during sputter deposition of ZnO Al onto CdS (left) and during evaporation of CdS onto ZnO A1 (right). The Fermi level in CdS is always within 1.8-2.2eV above the valence band maximum (Fermi level pinning)...

Fig. 8.10. Variation of k,r and krcc for hydrogen evolution on illuminated p-InP. The ratio k,J(k,r + km) represents the fraction of photogenerated electrons that are transferred across the interface. Note that k appears to depend weakly on potential. The non-ideal variation of km with potential is interpreted as evidence for partial Fermi level pinning.

Figure 19. Evolution of the density of states of an adsorbate in the presenee of a thin d band loealized near the Fermi level for an oxidation reaetion (left) and a reduetion reaetion (right).

Ultraviolet photoemission valance-band spectra with increasing Pd coverage on Al203/Re(0001) have been measured [33] and are shown in Figure 13. A gradual evolution of metallic valance bands with increasing cluster size is a manifestation of the increase in density of states near the Fermi level and the appearance of dispersing bands parallel and perpendicular to the substrate. The latter has been attributed to the formation of crystallites with a preferred orientation. The appearance of dispersion may also be used to dehne the boundary between a metallic and... 

Metal particles larger than about 100 atoms present an electronic band structure like in the bulk state, when the proportion of surface atoms, however, becomes non-negligible, several differences appear in the band structure. First, the width of the valence band is reduced and, second, its centre of gravity is shifted towards the Fermi level [55,56]. This evolution is a consequence of the reduction of the coordination that is equivalent to an increase in the localization of the valence electrons. This becomes more dramatic if we consider the local density of states on low-coordinated sites like edge and corner atoms. Figure 3.10 shows the calculated density of states on various atoms from a cubo-octahedron Pd cluster containing 3,871 atoms (equivalent to a radius of 5.7 nm) [40]. 

For a certain illumination intensity, the hole quasilevel Fp at the semiconductor surface can reach the level of an anodic reaction (reaction of semiconductor decomposition in Fig. 9). In turn, the electron quasilevel F can reach, due to a shift of the Fermi level, the level of a cathodic reaction (reaction of hydrogen evolution from water in Fig. 9). Thus, both these reactions proceed simultaneously, which leads eventually to photocorrosion. Hence, nonequilibrium electrons and holes generated in a corroding semiconductor under its illumination are consumed in this case to accelerate the corresponding partial reactions. 

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