Electrodes crystalline

If we look at the mechanistic and crystallographic aspects of the operation of polycomponent electrodes, we see that the incorporation of electroactive species such as lithium into a crystalline electrode can occur in two basic ways. In the examples discussed above, and in which complete equilibrium is assumed, the introduction of the guest species can either involve a simple change in the composition of an existing phase by solid solution, or it can result in the formation of new phases with different crystal structures from that of the initial host material. When the identity and/or amounts of phases present in the electrode change, the process is described as a reconstitution reaction. That is, the microstructure is reconstituted. 

TABLE 5-1 Characteristics of Solid-State Crystalline Electrodes ... 

Another consequence of pore geometry is that for crystalline electrodes, other crystal planes are exposed to the electrolyte at the pore tip than at the pore walls. The dependence of pore growth on crystal orientation of the silicon electrode is discussed in Chapters 8 and 9. 

Mackor, A., Blasse, G. 1981. Visible-light induced photocurrents in SrTiOj-LaCiOj single crystalline electrodes. Chem Phys Lett 77 6-8. 

B. Pettinger, C. Bdger, J. Lipkowski, Second harmonic generation anisotropy from single crystalline electrode surfaces in Interfacial Electrochemistry. Theory, Experiment and Applications (Ed. A. Wifckowski), Marcel Dekker, New York-Basel, 1999, p. 373. 

Photoexcitation of n-type semiconductors renders the surface highly activated toward electron transfer reactions. Capture of the photogenerated oxidizing equivalent (hole) by an adsorbed oxidizable organic molecule initiates a redox sequence which ultimately produces unique oxidation products. Furthermore, specific one electron routes can be observed on such irradiated surfaces. The irradiated semiconductor employed as a single crystalline electrode, as an amorphous powder, or as an optically transparent colloid, thus acts as both a reaction template and as a directed electron acceptor. Recent examples from our laboratory will be presented to illustrate the control of oxidative cleavage reactions which can be achieved with these heterogeneous photocatalysts. 

The use of porphyrins as sensitizers for the nano-crystalline electrode in DSSC is particularly attractive given their important role in photosynthesis and the relative ease modification on molecular structure. Not only the porphyrin monomers, but also the large porphyirn arrays have been tested extensively as sensitizers of wide-band-gap semiconductors like NiO, ZnO and Ti02 in the past several decades. The focus of this section is the various molecular structure modification of porphyrins for the purpose of applying as sensitizer in DSSCs. 

Aggregations of the porphyrin or phthalocyanine molecules on the surface of the nano-crystalline electrode always induce massive non-radioactive decay of the excited states and thus reduce the efficiency of the electron injection. Therefore, preventing the aggregation of the dye molecules on the electrode surface will be a foremost issue during the design of new sensitizers, which can be achieved normally by axial coordination, bulky substituents at peripheral positions or the co-adsorption with other materials. 

All three elements can scarcely be identified from impedance spectra of systems with adsorption for example, in many cases the adsorption resistance is masked by the diffusional element. In addition, whenever electrodes of inhomogeneous surfaces (e.g., poly crystalline electrodes) are measured, the resistance-diffusional element - capacitance terms of the adsorption impedance are smeared out yielding a CPE-like impedance. 

Table 21-2 lists some liquid-membrane electrodes available from commercial sources. The anion-sensitive electrodes shown make use of a solution containing an anion-exchange resin in an organic solvent. Liquid-membrane electrodes in which the exchange liquid is held in a polyvinyl chloride gel have been developed for Ca-, K", NOj, and BF4. These have the appearance of crystalline electrodes, which are considered in the following section. A homemade liquid-membrane ion-selective electrode is described in Feature 21-1. 

A crystalline electrode for fluoride ion is available from commercial sources. The membrane consists of a slice of a single crystal of lanthanum fluoride that has been doped with europium(II) fluoride to improve its conductivity. The membrane, supported between a reference solution and the solution to be measured, shows a theoretical response to changes in fluoride ion activity from 10 to 10 M. The electrode is selective for fluoride ion over other common anions by several orders of magnitude only hydroxide ion appears to offer serious interference. 

Heteroepitaxial layers of gold (100-150 nm thick) evaporated on cleaved mica give nearly monooriented crystalline electrodes with big (lll)-oriented grains (a few micrometers to a few tenths of a micron in area), nearly no azimuthal disorientations are observed at the surface. Their electrochemical behavior is close to that of massive conventional single crystals. ... 

In this work, we employed electrochemical AFM to investigate die surface structure of InSe and GaAs single-crystalline electrodes as these semiconductors have attractive properties described below but very limited information is available on the surface structure in electrolyte solution on a nanometer scale. 

Identification of the capacitance minimum with the PZC clearly suggests that the PZC depends upon the crystal face exposed to the solution. The capacitance of a model poly crystalline electrode, with the assumption that the surface was 46% (110), 23% (100) and 31% (111) is also shown. The PZC in this case is close to that of the (110) surface. The situation is more complicated with a real polycrystalline electrode surface with exposed high-index planes and defect sites. 

Potentiometry—the measurement of electric potentials in electrochemical cells—is probably one of the oldest methods of chemical analysis still in wide use. The early, essentially qualitative, work of Luigi Galvani (1737-1798) and Count Alessandro Volta (1745-1827) had its first fruit in the work of J. Willard Gibbs (1839-1903) and Walther Nernst (1864-1941), who laid the foundations for the treatment of electrochemical equilibria and electrode potentials. The early analytical applications of potentiometry were essentially to detect the endpoints of titrations. More extensive use of direct potentiometric methods came after Haber developed the glass electrode for pH measurements in 1909. In recent years, several new classes of ion-selective sensors have been introduced, beginning with glass electrodes more or less selectively responsive to other univalent cations (Na, NH ", etc.). Now, solid-state crystalline electrodes for ions such as F , Ag", and sulfide, and liquid ion-exchange membrane electrodes responsive to many simple and complex ions—Ca , BF4", CIO "—provide the chemist with electrochemical probes responsive to a wide variety of ionic species. 

The dependence of the adsorbate surface coverage 9 on the concentration of the adsorbate species in the solution ca is described by the adsorption isotherm. In the simplest case, that is, adsorption of a species with a single adsorption state on a structurally well-defined surface (e.g. a liquid or single-crystalline electrode), the adsorption can be usually described by the phenomenological Frumkin isotherm... 

Surface deconstruction has been discovered on single-crystalline electrodes in contact with electrolytes with the help of different experimental methods. These were the combination of cyclic voltammetry with ex situ analysis after... 

Second harmonic generation anisotropy from single-crystalline electrode suifaces [128] In-terfadal second harmonic generation (SHG) is an optical spechoscopy that is inherently surface-sensitive. All effects that modify the elechon density profile of the interface affect the SHG process. Therefore, SHG depends on the... 

On solid electrodes reliable former results suffered from the problem of reproducible surface preparation. A big step was made with the introduction of single crystalline electrodes and the reproducible preparation of single crystalline surfaces. The special problems connected with electrochemistry on single crystalline electrodes are described in a number of publications, e.g., by Clavilier et al ... 

---