Corrosion process semiconductor electrodes

Electrochemical properties of silicon single crystals, usually cuts of semiconductor wafers, have to be considered under two distinct respects (1) As an electrode, silicon is a source of charge carriers, electrons or positive holes, involved in electrochemical reactions, and whose surface concentration is a determining parameter for the rate of charge transfer. (2) As a chemical element, silicon material is also involved in redox transformations such as electroless deposition, oxide generation, and anodic etching, or corrosion processes. 

The practical success of semiconductor electrochemical photocells depends on how to prevent the photo-corrosion of the electrode materials. The various electrochemical processes at the... 

BDD electrodes are semiconductor electrodes with microcrystalline structure and relatively rough surfaces on the micrometric scale. Diamond-coated electrodes used for disinfection are chemically, mechanically, and thermally very resistant and show very low corrosion even under high electric charge. Diamond electrodes present no surface redox processes as known from other carbon electrodes (for example glassy carbon). 

Around 1975, investigations of photoelectrochemical reactions at semiconductor electrodes were begun in many research groups, with respect to their application in solar energy conversion systems (for details see Chapter 11). In this context, various scientists have also studied the problem of catalysing redox reactions, for instance, in order to reduce surface recombination and corrosion processes. Mostly noble metals, such as Pt, Pd, Ru and Rh, or metal oxides (RUO2) have been deposited as possible catalysts on the semiconductor surface. This technique has been particularly applied in the case of suspensions or colloidal solutions of semiconductor particles [101]. However, it is rather difficult to prove a real catalytic property, because a deposition of a metal layer leads usually to the formation of a rectifying Schottky junction at the metal-semiconductor interface (compare with Chapter 2), as will be discussed below in more... 

Related to the corrosion problems was a recent SECM study, which demonstrated the possibility of eliminating typical experimental problems encountered in the measurements of heterogeneous electron transfer at semiconductor electrodes (27). In this experiment, the redox reaction of interest (e.g., reduction of Ru(NH3)s+) is driven at a diffusion-controlled rate at the tip. The rate of reaction at the semiconductor substrate is probed by measuring the feedback current as a function of substrate potential. By holding the substrate at a potential where no other species than the tip-generated one would react at the substrate, most irreversible parasitic processes, such as corrosion, did not contribute to the tip current. Thus, separation of the redox reaction of interest from parallel processes at the semiconductor electrode was achieved. 

Our comprehensive understanding of materials corrosion fundamentals has advanced considerably over the decades. Modern corrosion science has made it clear that the corrosion process on metals and semiconductors consists of an anodic oxidation and a cathodic reduction both occurring across the material-aqua-solution interface. These reduction-oxidation reactions depend on the interfacial potential and hence on the electrode potential of materials. 

Passive films (corrosion) Photoredox processes with colloidal semiconductor particles as photocatalyst (e.g., degradation of refractory organic substances) Photoelectrochemistry (e.g., photoredox processes at semiconductor electrodes)... 

A schematic representation of the ideal electron-transfer rate and transfer coefficient as functions of potential for a semiconductor electrode is shown in Figure 18.2.6. Although there have been numerous studies with semiconductor electrodes, such ideal behavior is rarely seen (45, 47, 49, 57-59). Difficulties in such measurements include the presence of processes in parallel with the electron-transfer reaction involving dissolved reactant at the semiconductor surface, such as corrosion of the semiconductor material, effects of the resistance of the electrode material, and charge-transfer reactions that occur via surface states. 

The principles that govern electrochemistry at semiconductor electrodes can also be applied to redox processes in particle systems. In this case, one considers the rates of the oxidation and reduction half-reactions that occur on the particle, usually in terms of the current, as a function of particle potential. One can use current-potential curves to estimate the nature and rates of heterogeneous reactions on surfaces. This approach applies not only to semiconductor particles, but also to metal particles that behave as catalysts and to surfaces undergoing corrosion. 

A process on the semiconductor electrode is spontaneously set in depending on the comparative values of cp° for the useful reaction, and cpO jec corrosion of the electrode... 

With some types of cells corrosion was practically abolished, and service life up to one year was achieved for outdoor operation in the mid-Europe climat [20], Yet certain degradation processes other than corrosion (e g, ion exchange in the outmost layer of the semiconductor electrode [9]) restrict the service life of PEC cells which still are inferior to the solid-state solar cells whose service life comes to 10-20 years. 

On a short timescale (seconds to minutes), illuminated OCP analysis is a nondestructive technique for most materials. However, extended periods of illumination at open-circuit conditions may lead to corrosion of the photoelectrode surface [2]. Therefore, it is best to minimize the time of exposure to high-intensity illumination. Intense illumination can heat the solution (especially if the infrared radiation is not pre-filtered) at the electrode surface, which can cause a slow drift of the measured potential over the course of seconds or hours, depending on the rate of heating. In addition, drifts in the potential response may also be the result of corrosion processes or slow adsorption of cations or anions in solution to the semiconductor surface. A more rapid photovoltage response is often desirable and can be indicative of a better material. 

These limitations can be overcome by SECM. One of the important advantages of SECM is that the tip does not need to be a saniconductor. Therefore, any saniconductor electrode can be probed using a Pt UME tip, for example. Since very small currents are measured at the UME tip, the IR drop is minimal. Additionally, the UME tip quickly reaches a steady state due to the small area of the electrode so that effects due to double layer charging are avoided. Because the UME tip is poised at a potential specific to the redox couple used, corrosion processes taking place at semiconductor electrode do not contribute to the tip current. 

Several other attempts have been made by various authors to avoid anodic corrosion at n-type electrodes and surface recombination at p-type electrodes, by modifying the surface or by depositing a metal film on the electrode in order to catalyze a reaction. It has been frequently overlooked that the latter process leads to a semiconductor-metal junction (Schottky junction) which by itself is a photovoltaic cell (see Section 2.2) [14,27]. In the extreme case, only the metal contacts the redox solution. We have then a pure solid-state photovoltaic system which is contacting the solution via a metal. Accordingly, catalysis at the semiconductor electrode plays a minor role under these circumstances. 

The choice of a cell will be largely dependent on the type of processing to be accomplished. An example used in our laboratory for etching diffraction gratings is illustrated in Figure 1. The cell body should be machined from a polymer stable to the electrolyte of choice. Examples are nylon for many aqueous electrolytes and Teflon for HF-based etchants or nonaqueous media. The semiconductor electrodes must be mounted in such a way to prevent contamination of the ohmic contact (or other corrosive elements) by the electrolyte. In the approach illustrated in... 

The magnitude of the errors in determining the flat-band potential by capacitance-voltage techniques can be sizable because (a) trace amounts of corrosion products may be adsorbed on the surface, (b) ideal polarizability may not be achieved with regard to electrolyte decomposition processes, (c) surface states arising from chemical interactions between the electrolyte and semiconductor can distort the C-V data, and (d) crystalline inhomogeneity, defects, or bulk substrate effects may be manifested at the solid electrode causing frequency dispersion effects. In the next section, it will be shown that the equivalent parallel conductance technique enables more discriminatory and precise analyses of the interphasial electrical properties. 

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