Electrodes, semiconducting

This definition requires some explanation. (1) By interface we denote those regions of the two adjoining phases whose properties differ significantly from those of the bulk. These interfacial regions can be quite extended, particularly in those cases where a metal or semiconducting electrode is covered by a thin film. Sometimes the term interphase is used to indicate the spatial extention. (2) It would have been more natural to restrict the definition to the interface between an electronic and an ionic conductor only, and, indeed, this is generally what we mean by the term electrochemical interface. However, the study of the interface between two immiscible electrolyte solutions is so similar that it is natural to include it under the scope of electrochemistry. 

When a semiconducting electrode is brought into contact with an electrolyte solution, a potential difference is established at the interface. The conductivity even of doped semiconductors is usually well below that of an electrolyte solution so practically all of the potential drop occurs in the boundary layer of the electrode, and very little on the solution side of the interface (see Fig. 7.3). The situation is opposite to that on metal electrodes, but very similar to that at the interface between a semiconductor and a metal. 

Semiconducting electrodes offer the intriguing possibility to enhance the rate of an electron-transfer reaction by photoexcitation. There are actually two different effects Either charge carriers in the electrode or the redox couple can be excited. We give examples for both mechanisms. 

Tin oxide is a semiconductor with a wide band gap of Eg 3.7 eV, which can easily be doped with oxygen vacancies and chlorine acting as donor states. It is stable in aqueous solutions and hence a suitable material for n-type semiconducting electrodes. 

When all these factors contribute, the situation becomes almost hopelessly complicated. The simplest realistic case is that in which the photocarriers are generated in the space-charge region and migrate to the surface, where they are immediately consumed by an electrochemical reaction. We consider this case in greater detail. Suppose that light of frequency i/, with hu > Eg, is incident on a semiconducting electrode with unit surface area under depletion conditions (see Fig. 8.8). Let Iq be the incident photon flux, and a the absorption coefficient of the semiconductor at frequency v. At a distance x from the surface, the photon flux has decreased to Iq exp(—ax), of which a fraction a is... 

The above models describe pore formation from a mathematical point of view and the parameters of the models are subsequently assigned to physical values. The models described below are based on the specific chemistry or physics of the semiconducting electrode. 

In conclusion it should be emphasized that the passivation of the pore walls is electronic in nature. It therefore is not specific to silicon, but applies to all semiconducting electrodes. This is in contrast to chemical passivation, which is usually specific to a certain electrode material and electrolyte chemistry. 

Carbon nanomaterials as integrative materials in semiconducting electrodes... 

The Effect of Slow Electron Transfer Semiconducting Electrodes... 

Sn02 semiconducting electrodes are photosensitized by zinc porphyrin complexes but the zinc species is consumed in a competing dimerization reaction.1166... 

Several other varieties of semiconducting electrodes have been produced by chemical vapor deposition, as noted earlier, including films of Ti02 on Ti [69], Ti02 on plastic substrates [71], and other materials, such as Fe203 and Sn02 films [11]. Most of these other electrodes have been tested as electrodes for use in photoelectrochemically induced water splitting and have not found conventional electroanalytical utility. 

While many of the standard electroanalytical techniques utilized with metal electrodes can be employed to characterize the semiconductor-electrolyte interface, one must be careful not to interpret the semiconductor response in terms of the standard diagnostics employed with metal electrodes. Fundamental to our understanding of the metal-electrolyte interface is the assumption that all potential applied to the back side of a metal electrode will appear at the metal electrode surface. That is, in the case of a metal electrode, a potential drop only appears on the solution side of the interface (i.e., via the electrode double layer and the bulk electrolyte resistance). This is not the case when a semiconductor is employed. If the semiconductor responds in an ideal manner, the potential applied to the back side of the electrode will be dropped across the internal electrode-electrolyte interface. This has two implications (1) the potential applied to a semiconducting electrode does not control the electrochemistry, and (2) in most cases there exists a built-in barrier to charge transfer at the semiconductor-electrolyte interface, so that, electrochemical reversible behavior can never exist. In order to understand the radically different response of a semiconductor to an applied external potential, one must explore the solid-state band structure of the semiconductor. This topic is treated at an introductory level in References 1 and 2. A more complete discussion can be found in References 3, 4, 5, and 6, along with a detailed review of the photoelectrochemical response of a wide variety of inorganic semiconducting materials. 

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... 

Figure 4. Relative positions of hole decomposition potentials for the semiconductor and desired redox potentials for a photoelectrocheniical cell using n-type semiconducting electrodes. All examples are thermodynamically unstable, but (b) is kinetically more stable than (a).

Hole photoemission may also occur in appropriately biased PECs, although this process has not yet been observed. The major problem with the observation of photoemission from semiconducting electrodes is interference from the much larger photocurrents produced by the existence of the Schottky barrier at the interface. 

Mott-Schottky plot — is a graphical representation of the relationship between the -> space charge layer - capacitance, and the potential of a semiconducting -> electrode (Mott-Schottky equation) ... 

Novel Semiconducting Electrodes for the Photosensitized Electrolysis of Water Appears to be the first study on doping Ti02 to extend its light response into the visible range of the electromagnetic spectrum. 236... 

Volumes 26 and 27 are both concerned with reactions occurring at electrodes arising through the passage of current. They provide an introduction to the study of electrode kinetics. The basic ideas and experimental methodology are presented in Volume 26, whilst Volume 27 deals with reactions at particular types of electrode. Thus, Chapter 1 of the present volume deals with redox reactions at metal electrodes, Chapter 2 with semiconducting electrodes and Chapter 3 with reactions at metal oxide electrodes. Both theoretical aspects and experimental results are covered. 

M. G. Bradley and T. Tysak, p-type silicon based photoelectrochemical cells for optical energy conversion Electrochemistry of tetra-azomacrocyclic metal complexes at illuminated p-type silicon semiconducting electrodes, J. Electroanal. Chem. 135, 153, 1982. 

C. R. Cabrera and H. D. Abruna, Electrocatalysis of CO2 reduction at surface modified metallic and semiconducting electrodes, J. Electroanal. Chem. 209, 101, 1986. 

Dare-Edwards M. P., Goodenough J. B., Hamnett A., Seddon K. R. and Wright R. D. (1980), Sensitization of semiconducting electrodes with ruthenium-based dyes , Faraday Discussions, 285-298. 

In subsequent years semiconductive electrodes were more extensively studied titan before and atomic corrugation under in situ conditions was reported for more systems. 

The demonstration of nano fabrication on semiconductive electrodes was reported as early as in 1990 by Nagahara et aL These anthors fabricated nanostmctnres on Si(lOO) and GaAs(lOO) surfaces via local etching by maintaining a tip-surface bias and tunneling current. They observed nanostructures 20-nm wide and 1.5-nmdeep. 

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