N-Semiconductor-electrolyte interface

Figure 6. The energy level structure for an n-semiconductor-electrolyte interface as is appropriate for electron photoemission.

Figure 6.22 Schematic energy digram of a metal/electrolyte interface (a) and n-semiconductor/ electrolyte interface (b) for equilibrium 6) and deposition (ii) conditions., and denote...

Returning to the carrier collection problem, consider Fig. 18 for an n-semiconductor-electrolyte interface. As can be seen, the electron-hole pairs are optically generated, both in the field-free and in the space charge regions within the semiconductor. Recombination of these... 

Fig. 6 Schematic diagrams showing charge and potential distribution in an n-semiconductor-electrolyte interface, (a) Variation in charge distribution within the space charge region and in Gouy layer, (b) Variation in potential within the space charge region, and electrolyte. In Helmholtz layer it changes linearly, whereas in Gouy layer it decreases exponentially.

Figure Bl.28.10. Schematic representation of an illuminated (a) n-type and (b) p-type semiconductor in the presence of a depletion layer fonned at the semiconductor-electrolyte interface.

Zegenhagen J, Kazimirov A, Scherb G, Kolb D M, Smilgies D-M and Feidenhans l R 1996 X-ray diffraction study of a semiconductor/electrolyte interface n-GaAs(001)/H2S04( Cu) 1996 Surf. Sc/. 352-354 346-51... 

Semiconductor-electrolyte interface, photo generation and loss mechanism, 458 Semiconductor-oxide junctions, 472 Semiconductor-solution interface, and the space charge region, 484 Sensitivity, of electrodes, under photo irradiation, 491 Silicon, n-type... 

Fig. 5.60 The semiconductor/electrolyte interface (a) before equilibration with the electrolyte, (b) after equilibration with the electrolyte in the dark, and (c) after illumination. The upper part depicts the n-semiconductor and the lower the p-semiconductor...

Semiconductor - Electrolyte Interlace The electric field in the space charge region that may develop at the semiconductor electrolyte interface can help to separate photogenerated e /h 1 couples, effectively suppressing recombination. When a semiconductor is brought into contact with an electrolyte, the electrochemical potential of the semiconductor (corresponding to the Fermi level, Ey of the solid [50]) and of the redox couple (A/A ) in solution equilibrate. When an n-type semiconductor is considered, before contact the Ey of the solid is in the band gap, near the conduction band edge. After contact and equilibration the Ey will... 

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]. 

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 4.12 is an illustration of the potential distribution for n-type semiconductor particles at the semiconductor-electrolyte interface. There are two limiting cases of equation (4.8.11) for photo-induced electron transfer in semiconductors. For large particles the potential drop within the semiconductor is defined by ... 

Fig. 16. Energy diagram of a semiconductor (n-type)-electrolyte interface (a) in darkness, (b) under illumination.

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-... 

Below stepped-illumination experiments are presented for the photo-assisted electrolysis of water using n-type TiC or SnC photoanode/dark Pt cathode systems. An analysis of these results will be performed, focusing on the influence of the anodic halfcell reaction products upon the electronic state of the semiconductor /electrolyte interface. 

Figure 4. Energy diagram of a Schottky barrier formed at an n-type semiconductor-electrolyte interface...

We have performed an experimental study of photo-assisted electrolysis for illuminated n-type TiC>2 photoanode/dark Pt cathode systems. Analysis of these results indicates that the electronic state of the semiconductor/electrolyte interface is influenced by the electrolysis reaction products, in a manner not previously accounted for. 

Consider a photo-assisted water electrolysis cell, incorporating a photoanode and dark metal cathode. Illumination of the n-type semiconductor photoanode with a depletion space charge region results in a net flow of positive vacancies, or holes, to the semiconductor/electrolyte interface. Here the hole (h+) may be accepted by the reduced form of the oxygen redox couple. 

Unpinning of band edges at the semiconductor/electrolyte interface is understood as a common phenomenon for n- and p-type materials. Thus, the band edge positions as obtained from Hatband potential measurements in the dark, cannot be taken as a fixed value for the interpretation of charge transfer processes. More investigations in this direction are necessary. 

Tafalla, D. and Salvador, P. 1987. Mechanisms of charge transfer at the semiconductor-electrolyte interface oxygen electroreduction at naked and platinized n-Ti02 electrodes. Ber. Bunsenges. Phys. Chem., 91,475M79. 

Fig. 2 Hel photoelectron spectra of n-type a) and p-type (b) (0001) for increasing coverages by H20. energy correlation at the semiconductor/ adsorbate interface is shown in Fig. 1. It corresponds to the diagram of the semiconductor/electrolyte interface as is suggested by a comparison of contact potential differences and photopotentials obtained for the different halogens in UHV and in electrochemical junctions (organic electrolytes) (compare Table 1).

In the dark, the junction between an extrinsic (doped) semiconductor and a redox electrolyte behaves as a diode because only one type of charge carrier (electrons for n-type and holes for p-type) is available to take part in electron transfer reactions. The potential distribution across the semiconductor/electrolyte interface differs substantially from that across... 

Photoelectrochemistry — In principle, any process in which photon absorption is followed by some electrochemical process is termed photo electro chemical, but the term has come to have a rather restricted usage, partly to avoid confusion with photoemission (q.v.). The critical requirements for normal photo electro chemical activity is that the electrode itself should be a semiconductor that the electrolyte should have a concentration substantially exceeding the density of -> charge carriers in the semiconductor and that the semiconductor should be reverse biased with respect to the solution. To follow this in detail, the differences in potential distribution at the metal-electrolyte and semiconductor-electrolyte interfaces need to be understood, and these are shown in Fig. 1, which illustrates the situation for an n-type semiconductor under positive bias. 

---