Anodic Oxidation of Semiconductors

Silicon. The snrface of silicon immersed in fluoride media is of interest for semiconductor processing and production of porous silicon (Section 5.7) [541, 542, 549, 550]. A typical current-potential curve of p-Si in a flnoride electrolyte (0.975 MNH4CI + 0.025 MNH4F + 0.025 MHF, pH 2.8) measured at a rotating disk electrode at rotation rate of 3000 rpm and potential scanning speed of 5 mV s is shown in Fig. 7.29. The steep rise of the current density near 

ULTRATHIN FILMS AT GAS-SOLID, GAS-LIQUID, AND SOLID-LIQUID INTERFACES 

As discussed in Section 3.7, a smooth background that increases at lower wavenumbers is characteristic of free-carrier absorption, also termed Drude absorption (DA) (Figs. 3.44 and 3.47). The potential dependence of DA for the silicon-fluoride electrolyte interface is shown in Fig. 7.31. Free-carrier absorption is negligible at negative potentials, at which the silicon electrode is depleted. Since DA obeys the Schottky-Mott law (inset in Fig. 7.31), the flat-band potential was found to be - -0.4 V. Weak absorption by flee carriers 

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The method nearest to electronic device fabrication is the nanoscale processing based on the anodic oxidation of semiconductors and metals. The following electrochemical reactions proceed after applying voltage between the probe and the substrate in the column of adsorbed water generated at the region between them in the air as shown in Fig. 17. 

Fig. 9 and 10 give reaction schemes for the anodic oxidation of semiconductors. The energetics which control the reaction rates are schematically described in Fig.11. 

This chapter considers the fabrication of oxide semiconductor photoanode materials possessing tubular-form geometries and their application to water photoelectrolysis due to their demonstrated excellent photo-conversion efficiencies particular emphasis is given in this chapter to highly-ordered Ti02 nanotube arrays made by anodic oxidation of titanium in fluoride based electrolytes. Since photoconversion efficiencies are intricately tied to surface area and architectural features, the ability to fabricate nanotube arrays of different pore size, length, wall thickness, and composition are considered, with fabrication and crystallization variables discussed in relationship to a nanotube-array growth model. 

Relationships of other type are observed in the case where both the conjugated reactions proceed through the same band (Fig. 13b). For example, the cathodic reaction (42b) can take place with the participation of valence electrons rather than conduction electrons, as was assumed above. Thus, reduction of an oxidizer leads to the injection of holes into the semiconductor, which are used then in the anodic reaction of semiconductor oxidation. In other words, the cathodic partial reaction provides the anodic partial reaction with free carriers of an appropriate type, so that in this case corrosion kinetics is not limited by the supply of holes from the bulk of a semiconductor to its surface. Here the conjugated reactions are in no way independent ones. 

Anodic oxidation of pyrrole and N-substituted pyrroles results in the formation of polypyrroles in an oxidized state, which can be useful for the preparation of conducting organic polymers.185-188 Oxidation of 2,5-di-substituted pyrroles produces soluble products and no layer of polymers.187 One of the proposed applications of such a layer of conducting polymer is the protection of semiconductor electrodes from photocorrosion.189-191... 

The results of this kinetic analysis have been included in Table I. It can be seen that, if both the anodic decomposition of the semiconductor and the anodic oxidation of the competing reactant would occur by irreversible hole-capture steps ((L)(H)(I) or (M)(H)(1)), as was hitherto generally accepted, the stabilization should be independent of light intensity, in contradiction with the results described above. The mechanism in which the reducing agent reacts by donating an electron to a localized surface hole ((L)(X)) leads to an expression in which s is a function of the variable (y/j) only. The three other mechanisms considered lead to the relationship of the type (18), in which s is a function of (y2/j). 

Electrochemical reactions at metal electrodes can occur at their redox potential if the reaction system is reversible. In cases of semiconductor electrodes, however, different situations are often observed. For example, oxidation reactions at an illuminated n-type semiconductor electrode commence to occur at around the flat-band potential Ef j irrespective of the redox potential of the reaction Ergdox Efb is negative of Ere 0 (1 2,3). Therefore, it is difficult to control the selectivity of the electrochemical reaction by controlling the electrode potential, and more than one kind of electrochemical reactions often occur competitively. The present study was conducted to investigate factors which affect the competition of the anodic oxidation of halide ions X on illuminated ZnO electrodes and the anodic decomposition of the electrode itself. These reactions are given by Eqs 1 and 2, respectively ... 

In the development of photoelectrochemical (PEC) solar cells, one of the most difficult problems is the corrosion problem. In any solvent, but particularly in solvents with water present, anodic currents flowing from the solid to the solution will usually lead to corrosion. Specifically the corrosion will take the form of anodic oxidation of the semiconductor, with the products remaining as a film, dissolving into the solution, or evolving as a gas. Any such action will degrade the solar cell. 

Of particular interest in this context has been the finding that the Kolbe reaction, the anodic oxidation of carboxylic acids (Equation 1) (2), can be made to occur at n-type oxide semiconductor photoanodes to the virtual exclusion of oxygen formation (3,4,5). 

The multi-equivalent anodic oxidation of the semiconductor involves several consecutive electrochemical steps, the first of which can be symbolized as... 

In contrast to the processes described above, the electrooxidation of metals and alloys still cannot be considered as an accepted electrosynthetic method as yet only its principal possibilities have been demonstrated. At the same time, the anodic oxidation of transition metals, which forms the basis for a number of semiconductor technologies, is extremely effective and convenient for varying and controlling the thickness, morphology, and stoichiometry of oxide films [233]. It therefore cannot be mled out that, as the concepts concerning the anodic behavior of metal components of HTSCs in various media are developed, new approaches will be found. The development of combined methods that include anodic oxidation can also be expected, by analogy with hydrothermal-electrochemical methods used for obtaining perovskites based on titanium [234,235], even at room temperature [236]. 

Polyanilines (Scheme 36) are conjugated polymers whose it electrons are delocalized over the whole molecule. They are important conducting polymers that also act as semiconductors, in a similar manner to inorganic semiconductors121 m. They are made by chemical or electrochemical (anodic) oxidation of aniline. The product, a poor textile colorant, dates from the 1860s, and is still known by the name given at that time, emeraldine. In the electrochemical process, it is possible to produce thin films directly on conductive substrates. Polyanilines have been used in photoelectrochemical devices124-126. 

The anodic passivation of semiconductors in aqueous solution occurs in much the same way as that of metals and produces a passive oxide film on the semiconductor electrodes. Figure 22.25 shows the anodic dissolution current and the thickness of the passive film as a function of electrode potential for p-type and n-type silicon electrodes in basic sodium hydroxide solution [32,33], As mentioned earlier, silicon dissolves in the active state as divalent silicon ions and in the passive state a film of quadravalent insoluble silicon dioxide is formed on the silicon electrode. The passive film is in the order of 0.2-1.0 nm thick with an electric field of 106 107 V cm 1 in the film within the potential range where water is stable. 

The possible involvement of R in the Kolbe reaction has been regularly investigated using ESR but, to date, positive evidence for radical intermediates has been lacking apart from two cases, firstly the photo-Kolbe reaction at irradiated semiconductor electrodes [76] and, secondly, the anodic oxidation of triphenylacetic acid at platinum electrodes in acetonitrile. The latter will be discussed in this section. 

The application of photocurrent spectroscopy is not restricted to bulk semiconductors and insulator electrodes. The anodic oxidation of many metal electrodes produces surface films that are insulators or semiconductors, and in spite of the fact that these surface films are often very thin, their characterisation by photocurrent spectroscopy poses few experimental difficulties since photocurrents as small as 10 10 A can be measured by conventional lock-in methods. The instrumentation required for photocurrent spectroscopy is relatively modest and the technique is undemanding in terms of the degree of optical perfection of the electrode surface. Consequently, there seems to be considerable scope for the application of this type of spectroscopy to electrochemical problems such as corrosion, for example, where surface roughening may rule out methods that require an optically flat surface. 

Electrodes can be prepared with films of semiconductor particles. A straightforward approach involves suitable chemical treatment of a substrate metal, e.g., chemical or anodic oxidation of Ti to form a film of Ti02, which is made up of many small crystals, called grains. An alternative is to spread a film of semiconductor particles on an electrode surface. Films of nm-size semiconductor particles nanocrystalline films) have been of special interest. Such small particles (variously called quantum particles, Q-particles, or quantum dots), have properties that differ from those of larger (/xm) dimension (86, 87). 

In this section we use two types of complex charge-transfer reaction to illustrate the general approach to the elucidation of reaction mechanisms at single crystal electrodes. These reactions are photocurrent doubling at -type and p-type semiconductors and the (photo)anodic oxidation of the semiconductor itself. 

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