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can only be made by using well-defined single crystal electrodes. This additionally requires not only electrochemical methods of characterization, but also other methods supplying detailed structural information (LEED, STM, etc., see Section 5.5.6). Solid metal electrodes are usually polished mechanically and are sometimes etched with nitric acid or aqua regia. Purification of platinum group metal electrodes is effectively achieved also by means of high-frequency plasma treatment. However, electrochemical preparation of the electrode immediately prior to the measurement is generally most effective. The simplest procedure is to polarize the electrode with a series of cyclic voltammetric pulses in the potential range from the formation of the oxide layer (or from the evolution of molecular oxygen) to the potential of hydrogen evolution (Fig. 5.18F). The most important electrode material is platinum. Its widespread electrochemical use follows from its relatively high chemical inertness and electrocatalytic properties. The available potential window of platinum (the potential region where there is comparatively small faradaic current flow) is the interval from about 0 to +1.6 V versus SHE in acidic aqueous electrolytes; the negligible hydrogen overpotential of platinum makes it ideally suitable for the preparation of a standard hydrogen electrode (see Section 3.2.1). Platinum electrode is in aqueous electrolytes quickly covered, according to the polarization voltage, either with adsorbed hydrogen or surface oxide; only in a narrow potential interval (from 0.4 to 0.8 V versus SHE, the so-called 'double layer region') the Pt surface might be considered almost free from adsorbed or chemisorbed species. In practice, however, anions or organic impurities are always adsorbed even in this potential region. The Pt surface particularly interacts with double bonds in organic molecules. Chemisorption of electroactive olefinic molecules has also been employed for the preparation of chemically modified electrodes (see below).
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Platinum electrodes are made usually from polycrystalline metal; the crystal planes at the surface include both the (111) and (100) faces in approximately equal proportions. The electrochemical properties of Pt(lll) and Pt(100) faces are not identical. (Generally, the physical properties of individual metal crystal faces, such as work function, catalytic activity, etc., are different.) Cyclic voltammetry studies of single-crystal platinum electrodes in acidic aqueous electrolytes showed that the two characteristic peaks of hydrogen adsorption/desorption on platinum (see Fig. 5.40) correspond in fact to reactions at two different crystal faces: the peak at lower potential to Pt(lOO) and the other one to Pt(lll). The opposite case to Pt single crystal are electrodes whose surface area has been increased by covering with platinum black. This so-called platinized platinum electrode (see page 173) shows up to 1000 x higher physical surface area than the geometric surface area (the ratio of these two quantities is called roughness factor, see page 246). An increase in the physical surface area (roughness factor) of the electrode results in decreasing the relative coverage with surface-active impurities. The roughness factor of a single- crystal electrode is theoretically equal to one, but this value can hardly be achieved in view of microscopic imperfections of the real crystal face (see Fig. 5.24). A bright polycrystalline platinum electrode shows a roughness factor 1.3-3 depending on the level of polishing. The second most widely used noble metal for preparation of electrodes is gold. Similar to Pt, the gold electrode, contacted with aqueous electrolyte, is covered in a broad range of anodic potentials with an oxide film. On the other hand, the hydrogen adsorption/desorption peaks are absent on the cyclic voltammogram of a gold electrode in aqueous electrolytes, and the electrocatalytic activity for most charge transfer reactions is considerably lower in comparison with that of platinum. Semiconductors. In Sections 2.4.1, 4.5 and 5.10.4 basic physical and electrochemical properties of semiconductors are discussed so that the present paragraph only deals with practically important electrode materials. The most common semiconductors are Si, Ge, CdS, and GaAs. They can be doped to p- or n-state, and used as electrodes for various electrochemical and photoelectrochemical studies. Germanium has also found application as an infrared transparent electrode for the in situ infrared spectroelectrochemistry, where it is used either pure or coated with thin transparent films of Au or C (Section 5.5.6). The common disadvantage of Ge and other semiconductors mentioned is their relatively high chemical reactivity, which causes the practical electrodes to be almost always covered with an oxide (hydrated oxide) film. The surface reactivity is especially pronounced under illumination of the semiconductor electrode with photons of an energy greater than the band gap. The photogenerated minority carriers (electrons or holes) may react
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309 with the electrode material to cause its photodecomposition, usually called photocorrosion (for electrochemical corrosion, see Section 5.8.4). It appears with both p-type semiconductors (surface reduction), and n-type semiconductors (surface oxidation), but it is practically more important in the latter case. The photocorrosion reactions can be demonstrated, e.g., on Si and CdS as follows: (a) Cathodic photocorrosion of p-type materials p-Si + 4e" + 2 H 2 O ^ SiH4 + 4OH" (b) Anodic photocorrosion of n-type materials n-Si + 4h+ + 2 H 2 O ^ SiO2 + 4H + n-CdS + 2 h + ^ Cd2+ + S (5.5.26) (5.5.25)
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Here, the electrons are denoted e~ and the holes h + . The stability of n-Si, n-Ge, n-CdS, and n-GaAs photoanodes against photocorrosion increases in the presence of a solution species that reacts rapidly enough with photogenerated holes to compete with the surface oxidation. An alternative way is to cover the semiconductor electrode with an electronically conducting polymer film, such as polyvinylferrocene, polypyrrole, polyaniline, Nafion/tetrathiafulvalene, etc. These films are capable of efficiently capturing photogenerated holes and transport them to the solution, where they undergo a useful redox reaction (photoelectrosynthesis). The removal of photogenerated holes suppresses not only the photocorrosion, but also electron-hole recombination within the illuminated semiconductor, which increases the quantum yield of the photoelectrochemical process. Protection of p-type semiconductors by the mentioned conductive films is also possible; it enhances, for example, the rate of H2 evolution at semiconductor photocathodes. Various other semiconductor materials, such as CdSe, MoSe, WSe, and InP were also used in electrochemistry, mainly as n-type photoanodes. Stability against photoanodic corrosion is, naturally, much higher with semiconducting oxides (TiO2, ZnO, SrTiO3, BaTiO3, WO3, etc.). For this reason, they are the most important n-type semiconductors for photoanodes. The semiconducting metal oxide electrodes are discussed in more detail below. Metal oxides. Noble metals are covered with a surface oxide film in a broad range of potentials. This is still more accentuated for common metals, and other materials of interest for electrode preparation, such as semiconductors and carbon. Since the electrochemical charge transfer reactions mostly occur at the surface oxide rather than at the pure surface, the study of electrical and electrochemical properties of oxides deserves special attention.
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310 The electronic conductivity of metal oxides varies from values typical for insulators up to those for semiconductors and metals. Simple classification of solid electronic conductors is possible in terms of the band model, i.e. according to the relative positions of the Fermi level and the conduction/valence bands (see Section 2.4.1). A transition behaviour between a semiconductor and a metal can also be observed with metal oxides. This occurs when the Fermi level of the semiconductor is shifted (usually by heavy doping) into the conduction or valence band. This system is called degenerate semiconductor. If there exists a sufficient density of electronic states above and below thus shifted Fermi energy, the degenerate semiconductor can behave in contact with an electrolyte as a quasimetal electrode (e.g., with the charge transfer coefficient a close to 0.5, cf. Section 5.2.8). The quasimetallic behaviour of a degenerate semiconductor electrode is also conditioned by a sufficient capacity of the electronic states near the Fermi energy. Assuming, for simplicity, the double layer at the semiconductor/electrolyte interface as a parallel plate capacitor, whose capacity is independent of the potential difference across the electric double layer, the charge asc corresponding to a potential difference A|0 applied across the Helmholtz layer of thickness d is (cf. Section 4.5.1) osc = As2<t> 80/d (5.5.27)
If the charge asc is fully accommodated by electronic states near the Fermi energy, no space charge is formed in the electrode phase, and any voltage applied to the electrode appears exclusively across the Helmholtz layer, i.e. the system behaves like a metal. Degeneracy can be introduced not only by heavy doping, but also by high density of surface states in a semiconductor electrode (pinning of the Fermi level by surface states) or by polarizing a semiconductor electrode to extreme potentials, when the bands are bent into the Fermi level region. Another possibility of the appearance of a quasimetallic behaviour of metal oxides occurs with thin oxide films on metals. If the film thickness is sufficiently small (less than ca. 3 nm), the electrons can tunnel through the oxide film, and the charge transfer actually proceeds between the level of solution species and the Fermi level of the supporting metal (Fig. 4.12). Although the band model explains well various electronic properties of metal oxides, there are also systems where it fails, presumably because of neglecting electronic correlations within the solid. Therefore, J. B. Goodenough presented alternative criteria derived from the crystal structure, symmetry of orbitals and type of chemical bonding between metal and oxygen. This 'semiempirical' model elucidates and predicts electrical properties of simple oxides and also of more complicated oxidic materials, such as bronzes, spinels, perowskites, etc. There is a number of essentially non-conducting metal oxides acting as passive layers on electrodes; the best known example is A12O3. Metals that
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311 form insulating oxide films by anodic bias (e.g. Al, V, Hf, Nb, Ta, Mo) are termed 'valve metals' (cf. Section 5.8.3). Some insulating oxides become semiconducting by doping. This can be achieved either by inserting certain heteroatoms into the crystal lattice of the oxide, or more simply by its partial sub-stoichiometric reduction or oxidation, accompanied with a corresponding removal or addition of some oxygen anions from/into the crystal lattice. (Many metal oxides are, naturally, produced in these mixed-valence forms by common preparative techniques.) For instance, an oxide with partly reduced metal cations behaves as a n-doped semiconductor; a typical example is TiO2. Titanium dioxide is available as rutile (also in the form of sufficiently large single crystals) or anatase (only in polycrystalline form). Third modification, brookite, has no significance for electrode preparation. Pure rutile or anatase are practically insulators, with conductivities of the order of 10~13S/cm. Doping to n-type semiconducting state is performed, for example, by partial reduction with gaseous hydrogen, or simply by heating at temperatures about 500 C. This is accompanied by a more or less deep coloration of the originally white material, and by an increase of the electronic conductivity by orders of magnitude. Semiconducting titanium dioxide is one of the most important electrode materials in photoelectrochemistry (see Section 5.9.2). Another example of a non-stoichiometric, n-semiconducting oxide is PbO2. This has been extensively studied as a positive electrode material for lead/acid batteries. Lead dioxide occurs in two crystal modifications, orthorhombic ar-PbO2, and more stable tetragonal j3-PbO2; both polymorphs show high electronic conductivity of the order of 102 S/cm, and a typical mixed-valence Pb(II)/Pb(IV) composition with the O/Pb ratio in the range about 1.83-1.96. If the lead dioxide is prepared in an acidic aqueous medium (as in a lead/acid battery), the oxygen deficiency is partly introduced by replacement of O by OH, i.e. a more correct formula of this material is Pb(IV)(1_;t/2)Pb(II);c/2H;cO2 Both modifications of PbO2 occur in the lead/acid battery in the approximate ratio of a I(3 1/4. The standard potential of the ar-PbO2/PbSO4 redox couple in acidic medium is Eo= 1.697 V (for the / modification by about 10 mV lower). Although the lead dioxide is thermodynamically unstable in contact with water, its relatively high oxygen overvoltage is responsible for a reasonable kinetic stability. A PbO2 layer supported by Pb is suitable for anodic oxidations of various inorganic and organic substances, since it is stable against further oxidation and its ohmic resistance is negligible. Another class of conducting oxides are degenerate semiconductors, obtained by heavy doping with suitable foreign atoms. Two oxides, n-SnO2 (doped with Sb, F, In) and n-In2O3 (doped with Sn), are of particular interest. These are commercially available in the form of thin optically
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transparent films on glass (Nesa, Nesatron, Intrex-K, etc.). Typical layer thickness is several hundreds of nm, and resistivities of up to 5-20 Q/square. (Note: the unit 'Q/square' follows from the fact that the resistance of a uniform film on square support is independent of the size of the square, if we measure the resistance in direction parallel to the square.) The electrochemical properties of conducting SnO2 and related materials have been studied especially by T. Kuwana et al. Optically transparent SnO2 electrodes have found interesting applications in spectroelectrochemistry (see Section 5.5.6). Similar to PbO2, these electrodes exhibit relatively high oxygen overpotential and a good anodic stability, especially in acidic medium. Their thermal stability is also satisfactory, i.e. SnO2 can be utilized in electrolytes such as eutectic melts. The available potential window is, nevertheless, limited at the cathodic side by irreversible reduction of the oxide to the metal. High electrical conductivity is also attained in oxides with very narrow, partially filled conduction bands; the best known example is RuO2. This material has a conductivity of about 2-3 104S/cm at the room temperature, and metal-like variations with the temperature. Some authors consider RuO2 and similar oxides as true metallic conductors, but others describe them rather as n-type semiconductors. RuO2 is an important electrode material for industrial anodic processes. Special attention is deserved by the so-called dimensionally stable anodes (DSA) invented by H. B. Beer in 1968. These are formed by a layer of a microcrystalline mixture of TiO2 and RuO2 (crystallite size less thn 0.1 /im) on a titanium support (Fig. 5.26). This material is suitable as anode for chlorine and oxygen evolution at high current densities. For industrial chlorine production, it replaced the previously used graphite anodes. These
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