Poly-crystalline surfaces

As a furtlier example for tire meaning of ex situ investigations of emersed electrodes witli surface analytical teclmiques, results obtained for tire double layer on poly crystalline silver in alkaline solutions are presented in figure C2.10.3. This system is of scientific interest, since tliin silver oxide overlayers (tliickness up to about 5 nm) are fonned for sufficiently anodic potentials, which implies tliat tire adsorjDtion of anions, cations and water can be studied on tire clean metal as well as on an oxide covered surface [55, 56]. For tire latter situation, a changed... 

Recently, with the improvement achieved in the preparation and control of surfaces, a number of approaches have been devoted to the estimation of the pzc of pt(lll).140 197 210 211 These are summarized in Table 29 for convenience of the reader. The value recommended for pc-Pt is also reported for comparison. In three cases the pzc has been estimated indirectly and the value is strikingly close to the pzc of poly crystalline Pt. In view of the heterogeneity of Pt surfaces, this closeness is puzzling and suggests that the phenomenon used to estimate the pzc does not conform to the concept of zero charge. 

In this section, we will investigate the surface structure of the electrode in the potential range before a surface or bulk oxide starts forming, and will restrict ourselves to the adsorption of atomic oxygen only (not OH ) [Jacob and Scheffler, 2007]. Furthermore, in our simulations, we assume a single-crystal Pt(lll) electrode, which will be compared with the experimental CV curve (Fig. 5.9) for poly crystalline Pt. This simplification is motivated by the fact that our interest here is to describe the general behavior of the system only. 

The drop of the voltammetric crurent is associated with Pt surface oxidation, and the drop on the negative-going mn is due to Reaction (12.9) (surface poisoning by CO) and the Tafehan kinetics of Reaction (12.8). Further, the shift between curves in Fig. 12.13a and b indicates that in the potential range between 0.5 and 0.6 V, methanol oxidation occms with zero or low level atop CO smface intermediate. The amplitudes on Fig. 12.13 on both scans nearly equal to each other indicate a high level of preferential (111) crystallographic orientation of the poly crystalline Pt surface used for this work, as inferred from data in [Adzic et al., 1982]. 

The mechanism of anodic oxidation of CO at polycrystalline Au remains uncertain. Several groups have reported that the voltammetry of Au in acidic electrolytes is straightforward, with a well-formed oxidation wave/peak [Stonehart, 1966 Gibbs et al., 1977 Kita et al., 1985 Sun et al., 1999]. There is, however, no voltammetric evidence for the adsorption of CO on the Au surface, and spectroscopic studies indicate only a weak interaction of CO with poly crystalline Au surfaces in acidic solutions [Kunimatsu et al., 1986 Cuesta et al., 2003]. Moreover, there is little evidence for the formation of oxidizing species at the potential where the oxidation process is observed. Certainly, the oxidation of CO occurs at a potential over 500 mV less positive than that where bulk Au oxide is formed, and, indeed, the formation of this oxide strongly... 

As a function of the surface potential the electron work function for a given material depends on the state of the surface of that material (adsorption, the presence of surface compounds, etc.). For crystalline substances (see Table 3.1), various crystal faces have various electron work function values, which can be measured for single crystals. For poly crystalline substances, the final value of the electron work function depends on the contribution of the individual crystal faces to the entire area of the phase and the corresponding electron work functions the final value of the work function, however, is strongly dependent on the experimental method used for the measurement. 

Platinum electrodes are made usually from poly crystalline 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.)... 

Further improvement of the Prussian blue-based transducer presents two principal problems. First, Prussian blue layers are not mechanically stable, especially on smooth electrode surfaces because of their poly crystalline nature. Second, despite the low electrode potential used, the most powerful reductants like ascorbic acid still interfere with sensor response if present in excessive concentrations. 

For a typical poly crystalline material the concentration profile is divided into three segments. Near the original surface the tracer distribution will be characteristic of volume diffusion and show a typical bell shape. The average tracer concentration... 

Electronic Interaction on Poly crystalline Metal Surfaces... 

Since the electron work functions and the specific surface energies of different planes of the same crystal may have different values, it would be interesting to study the electronic interaction during adsorption of foreign molecules on monocrystals. Investigations of monocrystals, however, encounter many difficulties therefore, one has to restrict oneself in general to poly crystalline surfaces, which also give remarkable results because the force of interaction essentially depends on the nature of the metal and differs for the same metal from one species of adsorbed molecules to the other. 

Poly crystalline silicon (poly-Si) has been formed by the plasma-enhanced decomposition of dichlorosilane in argon at temperatures above 625 °C, a frequency of 450 kHz, and a total pressure of 27 Pa. Doped films have been deposited by the addition of phosphine to the deposition atmospheres (213). Approximately 1 atom % of chlorine was found in the as-deposited films. Annealing in nitrogen at temperatures above 750 °C caused chlorine to difluse from the film surface, grain growth to occur, and the film resistivity to drop. Such heat treatments were necessary to achieve integrated-circuit-quality films. 

It is well known that contact between the bottom of the tip and the sample surface will not be between two smooth, regular surfaces. In particular, the bottom of the tip may contain many asperities, and one of these asperities will serve as the probe. In STM experiments the most common tip is made from a tungsten poly-crystalline wire, and other tip materials are commonly transition metals (platinum, iridium, alloys) [21]. It is generally agreed today that only a very sharp tip with a single atom at its pinnacle is suitable to obtain atomic resolution on close-packed surfaces. But, such a tip is highly unstable. Therefore, the fabrication and characterization of defined tips, e.g. by field ion microscopy, have not been achieved, nor can it... 

The (1120) faces, dominant on poly crystalline samples, are characterized by highly shielded fivefold Fe3+ ions (5.43 A apart) and partial homopolar character. The situation is similar to that encountered on prismatic (1120) faces of (y-Cr20, after sintering under severe conditions. When probed with CO, Fe3+ ions exposed on the (1100) surfaces form weak Fe3+ CO adducts, characterized by a very modest shift of the C-0 stretching mode relative to the gas phase value (v = 2164-2165.5 cm A v = 20-22 cm-1). It is thus concluded that the local electric fields associated with Fe3+ ions at the surface of (1100) faces are very weak and similar to those reported for Mg2+ on MgO (001) and Cr3+ on (y-( r202 (1120) surfaces. 

For post-type DTA s in which thermocouple junctions measure the temperature of the container of the sample (e.g. platinum or poly crystalline alumina crucibles), good mechanical contact between the sample and the bottom of the crucible will improve instrument sensitivity to transformations. Surface contact may be optimized by using samples shaped to match the crucible, or finely crushed granules, as opposed to more spherical or odd-shaped chunks. Optimum mechanical contact minimizes the lag time between when a reaction occurs and when heat propagates to/from the point of temperature measurement, and the reaction is recorded. 

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. 

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