Iridium oxide

Iridium Oxide. Iridium dioxide [12030 9-8] coatings, typically used in combination with valve metal oxides, are quite similar in stmcture to those of mthenium dioxide coatings. X-ray diffraction shows the mtile crystal stmcture of the iridium dioxide scanning electron micrographs show the micro-cracked surface typical of these thermally prepared oxide coatings. 

The second form consists of Pt metal but the iridium is present as iridium dioxide. Iridium metal may or may not be present, depending on the baking temperature (14). Titanium dioxide is present in amounts of only a few weight percent. The analysis of these coatings suggests that the platinum metal acts as a binder for the iridium oxide, which in turn acts as the electrocatalyst for chlorine discharge (14). In the case of thermally deposited platinum—iridium metal coatings, these may actually form an intermetallic. Both the electrocatalytic properties and wear rates are expected to differ for these two forms of platinum—iridium-coated anodes. 

A dimensionally stable anode consisting of an electrically conducting ceramic substrate coated with a noble metal oxide has been developed (55). Iridium oxide, for example, resists anode wear experienced ia the Downs and similar electrolytic cells (see Metal anodes). 

Alloys with iridium Iridium alloys with platinum in all proportions, and alloys containing up to about 40% iridium are workable, although considerably harder than pure platinum. The creep resistance of iridium-platinum alloys is better than that of rhodium-platinum alloys at temperatures below 500°C. Their stability at high temperatures, however, is substantially lower, owing to the higher rate of formation of a volatile iridium oxide. 

For comparison, Battles et al. (15) determined the partial heats of sublimation of Pu02(g) and Pu0(g) above PuOi.33 over the temperature range 1937 to 2342 K by means of mass spectrometric measurements with Iridium effusion cells. The absence of Iridium oxides or Iridium species In the vapor phase Indicated that Iridium was nonreducing toward plutonia. The partial heats of sublimation calculated from the slopes of the temperature dependency data yielded values of 127.1 1.2 and 138.8 1.6 kcal/mol for Pu0(g) and Pu02(g) ... 

Iridium as an electrode material has received considerable attention in the last decade not only because of its excellent catalytic properties but also in relation to the electrochromic effect observed for anodic iridium oxide films (AIROF). Electrochromism of iridium was thought to be of technical relevance for display applications and triggered several studies of the electrochemical and optical properties of AlROFs [67, 85-88],... 

Thick anodic iridium oxide films are formed by repetitive potential cycling between properly chosen anodic and cathodic limits [89]. The coloration (bleaching) transition is reflected in the cyclic voltammogram by a significant increase (decrease) of the electrode pseudo-capacity at a potential around 0.7 Vsce in acid electrolytes. At potentials above 0.7 V the thick film appears dark blue, while below 0.7 V the film is almost clear. 

Fig. 26. Cyclic voltammogram of a thick anodic iridium oxide film (AIROF) in 0.5 mol L 1 H,S04. The reaction mechanism for coloration and Oz evolution is indicated.

The number of protons extracted from the film during coloration depends on the width of the potential step under consideration. As can be seen in the formulation of Fig. 26 an additional valence state change occurs at 1.25 Vsce giving rise to another proton extraction. The second proton exchange may explain the observation by Michell et al. [91] who determined a transfer of two electrons (protons) during coloration. Equation (5) is well supported by XPS measurements of the Ir4/ and Ols levels of thick anodic iridium oxide films emersed at different electrode potentials in the bleached and coloured state. Deconyolution of the Ols level of an AIROF into the contribution of oxide (O2-, 529.6 eV) hydroxide, (OH, 531.2 eV) and probably water (533.1 eV) indicates that oxide species are formed during anodization (coloration) on the expense of hydroxide species. The bleached film appears to be pure hydroxide (Fig. 27). 

Fig. 27. Deconvoluted Ols levels of a thick anodic iridium oxide film at different potentials. After [34],...

In order to explain the changing optical properties of AIROFs several models were proposed. The UPS investigations of the valence band of the emersed film support band theory models by Gottesfeld [94] and by Mozota and Conway [79, 88]. The assumption of nonstoichiometry and electron hopping in the model proposed by Burke et al. [87] is not necessary. Recent electroreflectance measurements on anodic iridium oxide films performed by Gutierrez et al. [95] showed a shift of optical absorption bands to lower photon energies with increasing anodic electrode potentials, which is probably due to a shift of the Fermi level with respect to the t2g band [67]. 

S.A.M. Marzouk, S. Ufer, R.P. Buck, T.A. Johnson, L.A. Dunlap, and W.E. Cascio, Electrodeposited iridium oxide pH electrode for measurement of extracellular myocardial acidosis during acute... 

Fabrication methods and conditions that determine the structure and composition of iridium oxide affect pH response characteristics of resultant pH sensors. A comparison... 

Although the redox reaction mechanisms of iridium oxide are still not clear, most researchers believe that the proton exchange associated with oxidation states of metal oxides is one of the possible pH sensing mechanisms [41, 87, 100, 105], During electrochemical reactions, oxidation state changes in the hydrated iridium oxide layer are... 

Depending on the fabrication techniques and deposition parameters, the pH sensitive slope of IrOx electrodes varies from near-Nemstian (about 59 mV/pH) to super-Nemstian (about 70mV/pH or higher). Since the compounds in the oxide layers are possibly mixed in stoichiometry and oxidation states, most reported iridium oxide reactions use x, y in the chemical formulas, such as lr203 xH20 and IrOx(OH)y. Such mixed oxidation states in IrOx compounds may induce more H+ ion transfer per electron, which has been attributed to causing super-Nerstian pH responses [41],... 

Considering the H+ dependent redox reaction between two oxidation states of the iridium oxide as the basis of the pH sensing mechanism, the electrode potential changes to the hydrogen ion concentration are expressed by Nemstian equation ... 

FIGURE 10.8 Two-dimensional pH distribution measured by an iridium oxide-based planar micro-pH electrode array after stimulation for only one minute. The electrode site in dark color is the Pt stimulating electrode. All other 15 electrodes are iridium oxide pH sensing electrodes. (Reproduced from [19], with permission from the Electrochemical Society, Inc.)... 

Simultaneous and continuous measurements of extracellular pH, potassium K+, and lactate in an ischemic heart were carried out to study lactic acid production, intracellular acidification, and cellular K+ loss and their quantitative relationships [6, 7], The pH sensor was fabricated on a flexible kapton substrate and the pH sensitive iridium oxide layer was electrodeposited on a planar platinum electrode. Antimony-based pH electrodes have also been used for the measurement of myocardial pH in addition to their application in esophageal acid reflux detection. 

D. Wipf, E Ge, T. Spaine, and J. Baur, Microscopic measurement of pH with iridium oxide microelectrodes. Anal. Chem. 72, 4921-4927 (2000). 

K.G. Kreider, M.J. Tarlov, and J.P. Cline, Sputtered thin-film pH electrodes of platinum, palladium, ruthenium, and iridium oxides. Sens. Actuators B. 28, 167-172 (1995). 

M.L. Hitchman and S. Ramanathan, Evaluation of iridium oxide electrodes formed by potential cycling as pH probes. Analyst 113, 35-39 (1988). 

S.A. Marzouk, Improved electrodeposited iridium oxide pH sensor fabricated on etched titanium substrates. Anal. Chem. 75, 1258—1266 (2003). 

A. Bezbaruah and T. Zhang, Fabrication of anodically electrodeposited iridium oxide film pH microelectrodes for microenvironmental studies. Anal. Chem. 74, 5726-5733 (2002). 

P. VanHoudt, Z. Lewandowski, and B. Little, Iridium oxide pH microelectrode. Biotechnol. Bioeng. 40, 601-608 (1992). 

Iridium dioxide, 79 608-609, 650 Iridium oxides, electrochromic materials, 6 579-580, 580t... 

Various metal oxides including Pt02, Ir02, Ru02, 0s02, etc., have been used to prepare solid-state pH electrodes. In particular, the iridium oxide (mainly Ir02) pH electrodes exhibit important advantages... 

Kato H, Asakura K, Kudo A (2003) Highly efficient water splitting into H2 and O2 over lanthanum-doped NaTaOs photocatalysts with high crystallinity and surface nanostructure. J Am Chem Soc 125 3082-3089 Iwase A, Kato H, Kudo A (2005) A novel photodeposition method in the presence of nitrate ions for loading of an iridium oxide cocatalyst for water splitting. Chem Lett 34, 946-947... 

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