The Semiconductor-Electrolyte Interface at Equilibrium

Fig. 7.12 Energy diagram for the semiconductor-electrolyte interface at equilibrium for different concentrations...

The Semiconductor-Electrolyte Interface at Equilibrium 9 Vacuum reference... 

Figure 9. Three situations for an n-type semiconductor-electrolyte interface at equilibrium showing overlap of the redox energy levels with the semiconductor conduction band (a) with surface states (b) and with the semiconductor valence band (c). A discrete energy level is assumed for the surface states as a first approximation.

FIGURE 1.3. Schematic illustration of the double layers in the semiconductor/electrolyte interface at an equilibrium condition. K is the potential drop across the space charge layer and Vh is the potential drop across the Helmholtz double layer. After Morrison. " ... 

Fig. 12 Three situations for a n-type semiconductor-electrolyte interface at equilibrium (a), under reverse bias (b), and under forward bias (c). The size of the arrows denotes the magnitudes of the current in the two (i.e. anodic and cathodic) directions.

As shown in Fig. 3.13(b) and 3.13(c) when ratio n/nsfl is less than or greater than 1 the system is in non-equilibrium resulting in a net current, with the electron transfer kinetics at the semiconductor-electrolyte interface largely determined by changes in the electron surface concentration and the application of a bias potential. Under reverse bias voltage, Vei > 0 and ns,o > ns as illustrated in Fig. 3.13(b), anodic current will flow across the interface enabling oxidized species to convert to reduced species (reduction process). Similarly, under forward bias, Ve2 < 0 and ns > ns,o as illustrated in Fig. 3.13(c), a net cathodic current will flow. 

Figure 31 contains a schematic representation of the nanocrystalline semiconductor film-electrolyte interface at equilibrium (Figure 31a) and the corresponding situation under bandgap irradiation of the semiconductor (Figure 31b) [9]. Since the difiFusion length of the photogenerated carriers is usually larger than the physical dimensions of the structural units, holes and electrons can reach the impregnated electrolyte phase before they are lost via bulk recombination. This contrasts the situation with the single-crystal cases discussed earlier.

Kinetic or activation overpotential is defined as the potential in excess of the thermodynamic equilibrium ( °) potential of the half-reaction that is necessary to drive the reaction, and often plays an important role in determining overall reaction rates. A means of increasing the reaction rate is to specifically improve the kinetics of the surface reactions by the addition of catalyst materials which can reduce these overpotentials. In metal electrodes, the modification of the surface with a catalyst increases the current at a given applied potential. Similarly in photoelectrodes, addition of a catalyst can ensure efficient catalytic turnover of photogenerated minority charge carriers at the semiconductor-electrolyte interface. 

The f/fb is one of the most important quantities for semiconductor electrodes because it determines the band edge positions at the semiconductor-electrolyte interface, which in turn, determine the energies of conduction-band electrons and valence-band holes reacting with the electrolyte solution. It is known that l/fb for most semiconductors, such as n- and p-GaAs, GaP, InP, n-ZnO, n-Ti02, and n-Sn02, in aqueous electrolytes is solely determined by the solution pH and shifts in proportion to pH with a ratio of —0.059 V/pH [1,2]. This is explained by an adsorption equilibrium for H+ or OH at the semiconductor-electrolyte interface, for example,... 

Scheme of the energetic levels at the semiconductor-electrolyte interface for an n-type semiconductor (a) at equilibrium (b) flat band potential. 

The development of photocathode materials for either single- or dual-absorber cells has also received considerable attention [80, 101, 102]. Thermodynamic equilibrium dictates that p-type semiconductors will exhibit upward band bending when in contact with a liquid electrolyte. This behaviour is the opposite to that of n-type semiconductors described previously, and will result in the movement of photogenerated electrons towards the semiconductor-electrolyte interface while the holes are driven into the bulk of the electrode, towards the electrical back contact. At the surface, provided that the energy carried by the electrons is sufficient, H2 is evolved. As discussed previously, one of the electronic properties of metal oxides that makes them suitable for water photo-oxidation purposes is the O 2p character of the valence electrons, which places the VB edge at potentials... 

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