Metal n-type semiconductor

Fig. 2.19 Metal-n-type semiconductor junction (cf>m >

Schematic energy band diagram of the metal/n-type semiconductor interface according to interface gap state models. (From Cowley, A.M. and Sze, S.M., /. Appl. Phys., 36,3212,1965. With permission.)...

Energy-band diagram of the band bending interface in the presence of surface states for a metal/n-type semiconductor (a) under flat band condition and (b) after contact formation. 

Figure 2.3 Energy diagram of a metal-n-type semiconductor interface before and after contact (a) without surface states (b) with surface states.

Figure8.1 A metal n-type semiconductor pair before (a) and after (b) contact with no surface/interface states. The metal work function is greater than that for the semiconductor (0m>0s)-...

Figure 8.3 Ideal metal n-type semiconductor contacts under (a) equilibrium and (b) forward and (c) reverse biases. Also shown is the image-force lowering of the barrier.

In semiconductors, which have a bandgap, recombination of the excited carriers— return of the electrons from the conduction band to vacancies in the valence band—is greatly delayed, and the lifetime of the excited state is much longer than in metals. Moreover, in n-type semiconductors with band edges bent upward, excess electrons in the conduction band will be driven away from the surface into the semiconductor by the electrostatic held, while positive holes in the valence band will be pushed against the solution boundary (Fig. 29.3). The electrons and holes in the pairs produced are thus separated in space. This leads to an additional stabihzation of the excited state, to the creation of some steady concentration of excess electrons in the conduction band inside the semiconductor, and to the creation of excess holes in the valence band at the semiconductor-solution interface. 

The electrons produced in the conduction band as a result of illumination can participate in cathodic reactions. However, since in n-type semiconductors the quasi-Fermi level is just slightly above the Fermi level, the excited electrons participating in a cathodic reaction will almost not increase the energy effect of the reaction. Their concentration close to the actual surface is low hence, it will be advantageous to link the n-type semiconductor electrode to another electrode which is metallic, and not illuminated, and to allow the cathodic reaction to occur at this electrode. It is necessary, then, that the auxiliary metal electrode have good catalytic activity toward the cathodic reaction. 

Similar photovoltaic cells as those described above can be made with semiconductor/ liquid Junctions. The basic function of such a cell is illustrated in terms of an energy scheme in Fig. 2. The system consists of an n-type semiconductor and an inert metal... 

Fig. 15. Schematic energy diagram of the n-type semiconductor electronic bands at the solid/liquid interface modulated by discontinuous metal coating ...

Various other semiconductor materials, such as CdSe, MoSe, WSe, and InP were also used in electrochemistry, mainly as n-type photoanodes. Stability against photoanodic corrosion is, naturally, much higher with semiconducting oxides (Ti02, ZnO, SrTi03, BaTi03, W03, etc.). For this reason, they are the most important n-type semiconductors for photoanodes. The semiconducting metal oxide electrodes are discussed in more detail below. 

High electrical conductivity is also attained in oxides with very narrow, partially filled conduction bands the best known example is Ru02. This material has a conductivity of about 2-3 104S/cm at the room temperature, and metal-like variations with the temperature. Some authors consider Ru02 and similar oxides as true metallic conductors, but others describe them rather as n-type semiconductors. 

Semiconductor electrodes seem to be attractive and promising materials for carbon dioxide reduction to highly reduced products such as methanol and methane, in contrast to many metal electrodes at which formic acid or CO is the major reduction product. This potential utility of semiconductor materials is due to their band structure (especially the conduction band level, where multielectron transfer may be achieved)76 and chemical properties (e.g., C02 is well known to adsorb onto metal oxides and/ or noble metal-doped metal oxides to become more active states77-81). Recently, several reports dealing with C02 reduction at n-type semiconductors in the dark have appeared, as described below. 

Thus, although the potential required for polarization would be much larger at n-type semiconductors than at illuminated p-type semiconductors and despite the fact that not all n-type semiconductors can be used because of corrosion (or reduction) of semiconductor materials themselves, the use of n-type semiconductors to examine C02 reduction seems to be indicated because the cathodic current is much larger (the electron is the major carrier for n-type semiconductors), approaching that of metal electrodes, compared to the photocurrent obtained at illuminated p-type semiconductors,... 

Energy level diagram for an n-type semiconductor-metal photoelectrolysis cell in which the flat-band potential lf(b lies above the H+/H2 potential, whereas the 02/H20 potential lies above the valence band of the n-type semiconductor. 

The Schottky-Mott theory predicts a current / = (4 7t e m kB2/h3) T2 exp (—e A/kB 7) exp (e n V/kB T)— 1], where e is the electronic charge, m is the effective mass of the carrier, kB is Boltzmann s constant, T is the absolute temperature, n is a filling factor, A is the Schottky barrier height (see Fig. 1), and V is the applied voltage [31]. In Schottky-Mott theory, A should be the difference between the Fermi level of the metal and the conduction band minimum (for an n-type semiconductor-to-metal interface) or the valence band maximum (for a p-type semiconductor-metal interface) [32, 33]. Certain experimentally observed variations of A were for decades ascribed to pinning of states, but can now be attributed to local inhomogeneities of the interface, so the Schottky-Mott theory is secure. The opposite of a Schottky barrier is an ohmic contact, where there is only an added electrical resistance at the junction, typically between two metals. 

The low-pressure region displays the electroneutrality equation approximation [e ] = 2[Vx ]. Electrons predominate so that the material is an n-type semiconductor in this regime. In addition, the conductivity will increase as the partial pressure of the gaseous X2 component decreases. The number of nonmetal vacancies will increase as the partial pressure of the gaseous X2 component decreases, and the phase will display a metal-rich nonstoichiometry opposite to that in the high-pressure domain. Because there is a high concentration of anion vacancies, easy diffusion of anions is to be expected. 

Shallow levels play an important part in electronic conductivity. Shallow donor levels lie close to the conduction band in energy and liberate electrons to it to produce n-type semiconductors. Interstitial metal atoms added to an insulating ionic oxide often act in this way because metal atoms tend to ionize by losing electrons. When a donor level looses one or more electrons to the conduction band, it is said to be ionized. The energy level representing an ionized donor will be lower than that of the un-ionized (neutral) donor by the same amount as required to move the electron into the conduction band. The presence of shallow donor levels causes the material to become an w-type semiconductor. 

To understand the role of the noble metal in modifying the photocatalysts we have to consider that the interaction between two different materials with different work functions can occur because of their different chemical potentials (see [200] and references therein). The electrons can transfer from a material with a high Fermi level to another with a lower Fermi level when they contact each other. The Fermi level of an n-type semiconductor is higher than that of the metal. Hence, the electrons can transfer from the semiconductor to the metal until thermodynamic equilibrium is established between the two when they contact each other, that is, the Fermi level of the semiconductor and metal at the interface is the same, which results in the formation of an electron-depletion region and surface upward-bent band in the semiconductor. On the contrary, the Fermi level of a p-type semiconductor is lower than that of the metal. Thus, the electrons can transfer from the metal to the semiconductor until thermodynamic equilibrium is established between the two when they contact each other, which results in the formation of a hole depletion region and surface downward-bent band in the semiconductor. Figure 12.6 shows the formation of semiconductor surface band bending when a semiconductor contacts a metal. 

Figure S-4S shows the polarization curves observed, as a function of the film thickness, for the anodic and cathodic transfer reactions of redox electrons of hydrated ferric/ferrous cyano-complex particles on metallic tin electrodes that are covered with an anodic tin oxide film of various thicknesses. The anodic oxide film of Sn02 is an n-type semiconductor with a band gap of 3.7 eV this film usually contains a donor concentration of 1x10" ° to lxl0 °cm °. For the film thicknesses less than 2.5 nm, the redox electron transfer occurs directly between the redox particles and the electrode metal the Tafel constant, a, is close to 0.5 both in the anodic and in the cathodic curves, indicating that the film-covered tin electrode behaves as a metallic tin electrode with the electron transfer current decreasing with increasing film thickness.

For illustrations, we compare the transfer of anodic holes at metal electrodes and the transfer of anodic photoexdted holes at n-type semiconductor electrodes for the oxygen redox reaction shown in Eqn. 10-16 ... 

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