Solid-state crystal electrodes

Solid-state crystal electrodes have a life of about 1-2 years. However, if used at high temperatures, their life gets shortened considerabfy (1-3 months). Figure 2.3 is a diagrammatic representation of the crystal electrode. 

We have briefly encountered the solid-state fluoride electrode, which has a fully nemstian response down to c. 10 mol dm . The fluoride electrode is iiiunersed in a test solution of fluoride ion (usually aqueous), and the emf is then determined. At its heart is a single crystal of lanthanum fluoride doped with erbium fluoride, (see Figure 3.10). Like the pH electrode, a full fluoride electrode also contains a small reference electrode, meaning that a fluoride electrode is in reality a cell. The fluoride electrode does not suffer from interference from CP, so an AgCl Ag reference is the normal choice owing to its convenience and compact size. 

The liquid-membrane electrode is another important type of ion-selective electrode. The internal filling solution contains a source of the ion under investigation, i.e. one for which the ion exchanger is specific, while also containing a halide ion to allow the reference electrode to function. The physico-chemical behaviour of the ISE is very similar to that of the fluoride electrode, except that ise and the selectivity are dictated by the porosity of a membrane rather than by movement through a solid-state crystal. 

Considerable work has been devoted to the development of solid membranes that are selective primarily to anions. The solid-state membrane can be made of single crystals, polycrystalline pellets, or mixed crystals. The resulting solid-state membrane electrodes have found use in a great number of analytical applications. 

Single crystal and solid-state membrane electrodes... 

Ion-Selective Microelectrodes. Ion-selective electrodes can be fabricated in microassemblies for use in such applications as measuring ion activities in microsamples (10" ml) and as direct probes in tissues, tubules, capillaries, or even within individual cells. Microelectrodes can be fabricated with sensors made of glass, liquid ion-exchanger, or solid-state crystals [11,13]. 

Solid-state sensors for chloride, iodide, and fluoride are based on the solubility product of silver chloride or silver iodide particles in silicone rubber and a doped lanthanum fluoride single crystal, respectively. The fluoride-selective electrode was applied for the analysis of urine and bone tissue of people exposed to industrial sources as well as for control of therapeutic fluoride application for osteoporosis, whereas the chloride-selective sensor was applied to the analysis of sweat for the diagnosis of cystic fibrosis. In solid-state contact electrodes the solvent polymeric membrane is directly contacted to the solid field transducing element, although the reference electrode is separated from the ion-selective sensing pad. 

The potentiometric detection of sulfate can be performed by Gran s plot titration of the sulfate solution with a commercially available barium-selective electrode (17), Sulfate-selective electrodes based on solid state crystals have also been reported (18), More recently, polymeric-membrane sulfate-selective electrodes have been developed using imidazole and thiourea derivatives as ionophores (19, 20),... 

Silicon is prepared commercially by heating silica and carbon in an electric furnace, using carbon electrodes. Several other methods can be used for preparing the element. Amorphous silicon can be prepared as a brown powder, which can be easily melted or vaporized. The Gzochralski process is commonly used to produce single crystals of silicon used for solid-state or semiconductor devices. Hyperpure silicon can be prepared by the thermal decomposition of ultra-pure trichlorosilane in a hydrogen atmosphere, and by a vacuum float zone process. 

In the ceramics field many of the new advanced ceramic oxides have a specially prepared mixture of cations which determines the crystal structure, through the relative sizes of the cations and oxygen ions, and the physical properties through the choice of cations and tlreh oxidation states. These include, for example, solid electrolytes and electrodes for sensors and fuel cells, fenites and garnets for magnetic systems, zirconates and titanates for piezoelectric materials, as well as ceramic superconductors and a number of other substances... 

The energy-dispersive (EDX) solid state detector (SSD, Figs 4.6, 4.7) is made of lithium-drifted Si crystal (Si(Li)). Between a thin p-type and an n-type layer lies a high-resistivity Si crystal of centimeter dimensions. The front and end planes of the crystal are coated with Au and serve as electrodes. The crystal, cooled to 77 K by liquid nitrogen, represents a p-i-n diode (Fig. 4.7). An incident X-ray photon with... 

Yang YJ, He LY (2006) Dissolution of lead electrode and preparation of rod-shaped PbS crystals in a novel galvanic ceU. J Solid State Electrochem 10 430-433... 

Levi E, Lancry E, Gofer Y, Aurbach D (2006) The crystal structure of the inorganic surface films formed on Mg and Li intercalation compounds and the electrode performance. J Solid State Electrochem (2006) 10 176-184... 

Prieto A, Hernandez J, Herrero E, Feliu JM. 2003. The role of anions in oxygen reduction in neutral and basic media on gold single-crystal electrodes. J Solid State Electrochem 7 599-606. 

Water is involved in most of the photodecomposition reactions. Hence, nonaqueous electrolytes such as methanol, ethanol, N,N-d i methyl forma mide, acetonitrile, propylene carbonate, ethylene glycol, tetrahydrofuran, nitromethane, benzonitrile, and molten salts such as A1C13-butyl pyridium chloride are chosen. The efficiency of early cells prepared with nonaqueous solvents such as methanol and acetonitrile were low because of the high resistivity of the electrolyte, limited solubility of the redox species, and poor bulk and surface properties of the semiconductor. Recently, reasonably efficient and fairly stable cells have been prepared with nonaqueous electrolytes with a proper design of the electrolyte redox couple and by careful control of the material and surface properties [7], Results with single-crystal semiconductor electrodes can be obtained from table 2 in Ref. 15. Unfortunately, the efficiencies and stabilities achieved cannot justify the use of singlecrystal materials. Table 2 in Ref. 15 summarizes the results of liquid junction solar cells prepared with polycrystalline and thin-film semiconductors [15]. As can be seen the efficiencies are fair. Thin films provide several advantages over bulk materials. Despite these possibilities, the actual efficiencies of solid-state polycrystalline thin-film PV solar cells exceed those obtained with electrochemical PV cells [22,23]. 

In solid-state electrodes the membrane is a solid disc of a relatively insoluble, crystalline material which shows a high specificity for a particular ion. The membrane permits movement of ions within the lattice structure of the crystal and those ions which disrupt the lattice structure the least are the most mobile. These usually have the smallest charge and diameter. Hence, only those ions that are very similar to the internal mobile ions can gain access to the membrane from the outside, a feature that gives crystal membranes their high specificity. When the electrode is immersed in the sample solution, an equilibrium is established between the mobile ions in the crystal and similar ions in the solution and the resulting potential created across the membrane can be measured in the usual manner. 

The simplest solid-state membranes are designed to measure test ions, which are also the mobile ions of the crystal (first-order response) and are usually single-substance crystals (Figure 4.11). Alternatively, the test substance may be involved in one or two chemical reactions on the surface of the electrode which alter the activity of the mobile ion in the membrane (Figures 4.12 and 4.13). Such membranes, which are often mixtures of substances, are said to show second- and third-order responses. While only a limited number of ions can gain access to a particular membrane, a greater number of substances will be able to react at the surface of the membrane. As a result, the selectivity of electrodes showing second- and third-order responses is reduced. 

Ion hopping is a familiar concept in the chemistry of solid-state conductors, e.g. in the semiconductor industry. In the fluoride electrode, fluoride vacancies in.side the solid LaF lattice allow for conduction of charge (see Figure 3.11), in turn registered by the electrode as a potential. The emf is zero if the internal and external solutions are the same because the same numbers of fluoride ion enter the crystal from either face. 

As shown in Fig. 2.5, the cyclic voltammograms for Prussian blue attached to paraffin-impregnated graphite electrodes (PIGEs) in contact with aqueous electrolytes exhibit two well-defined one-electron couples. Prussian blue crystals possess a cubic structure, with carbon-coordinated Fe + ions and nitrogen-coordinated Fe + ions, in which potassium ions, and eventually some Fe + ions, are placed in the holes of the cubes as interstitial ions. The redox couple at more positive potentials can be described as a solid-state process involving the oxidation of Fe + ions. Charge conservation requires the parallel expulsion of K+ ions [77] ... 

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