Polycrystalline metal surfaces

This reported pzc is for polycrystalline Au. Electrochemists may debate the precise meaning of the pzc for a polycrystalline metal surface. However, for our purposes here it is the potential at which there is no net excess charge on the polycrys-taUine metal surface. 

Tables I-VI summarize the more reliable results for the adsorption of alkali metals and gases on polycrystalline metal surfaces. Many of the older...

Table 1.1 Values of the work function for polycrystalline metal surfaces [11, 12]...

The results of studies of copper surfaces by low-temperature adsorption isotherms may be summarized as follows. True surface areas of metallic specimens as small as 10 sq. cm. can be derived with a precision of 6% from low-temperature adsorption isotherms using vacuum microbalance techniques. This method is of special value in determining the average thickness of corrosion films formed by the reaction of gases or liquids with solids. The effect of progressive oxidation of a rough polycrystalline metal surface is to decrease the surface area to a point where the roughness factor approaches unity. 

Variation of heats of adsor-ption on different major crystal faces of copper. The effect of the crystal face of the adsorbent on physical adsorption has been treated theoretically by Barrer (153) for covalent surfaces and by Orr (151) and by Lenel (154) for dielectric surfaces. The vertical interactions between a nonpolar molecule and a polycrystalline metal surface have been independently treated by Lennard-Jones (155), by Bardeen (156), and by Margenau and Pollard (157). Considering the theoretical limitations involved in the last treatment, the observed agreement between the values calculated theoretically and the experimental values is acceptable. At present no explicit theoretical treatment of the physical adsorption of a nonpolar gas molecule on a single crystal metal surface in terms of the crystal parameter and geometry of the latter is available. 

Sulfur adsorption stoichiometries at saturation coverage for single-crystal surfaces of Pt (84, 85), Fe (72, 142), Mo (75,143, 144), Ag (53-56), and Cu (58,65), and for polycrystalline metal surfaces of Pt (145), Fe (101), Co (101), and Ru (101) have been reported. The general features observed for these metals are similar to those observed for Ni accordingly, only the more interesting observations will be discussed. 

In order to determine the electronic work function the metal must be ultrapure with no contamination on the surface. The biggest problem in this regard is the fact that most metals have a surface oxide layer under ambient conditions. A clean metallic surface can be formed by sputtering the metal in a vacuum chamber, and techniques are available for growing thin metal films of known crystallographic orientation. Otherwise, the metal surface may be cleaned by positive ion bombardment techniques at very low pressures. Values of the electronic work function for polycrystalline metal surfaces are summarized in table 8.2. The lowest... 

Table 8.2 Values of the Electronic Work Function for Polycrystalline Metal Surfaces ...

UPD is also observed on polycrystalline metal surfaces but only a broad peak is observed with no details about the different surface positions. The reason for this is the inhomogeneous surface structure of polycrystalline electrodes. 

Formation of a 2D phase on polycrystalline metal surfaces is rarely if ever observed, because the metal surface is not atomically flat. Crystallites could be at different angles with respect to each other and different crystal faces may be exposed to the electrolyte at random positions, preventing long-range order. Nevertheless, in some cases the surface of a polycrystalline metal is found to... 

The work function O is the energy required to remove an electron from a metal. It is an intrinsic property of the metal, which is measured under conditions of ultrahigh vacuum, on surfaces that have been meticulously cleaned. Even a small amount of impurity adsorbed on the surface (e.g. oxygen or an oxide, water, or carbonaceous molecule), could give rise to significant errors in the measured value of the work function. Moreover, the value measured on a polycrystalline metal surface is a weighted average of the contribution of the work function for different crystal faces. Thus, each metal has in fact several work function values, each characteristic of a different crystal face. The relevant scientific literature is replete with data with values of for different metals and many of the common crystal faces for each metal. 

The rate (or kinetics) and form of a corrosion reaction will be affected by a variety of factors associated with the metal and the metal surface (which can range from a planar outer surface to the surface within pits or fine cracks), and the environment. Thus heterogeneities in a metal (see Section 1.3) may have a marked effect on the kinetics of a reaction without affecting the thermodynamics of the system there is no reason to believe that a perfect single crystal of pure zinc completely free from lattic defects (a hypothetical concept) would not corrode when immersed in hydrochloric acid, but it would probably corrode at a significantly slower rate than polycrystalline pure zinc, although there is no thermodynamic difference between these two forms of zinc. Furthermore, although heavy metal impurities in zinc will affect the rate of reaction they cannot alter the final position of equilibrium. 

The quantities jXe, pe >

bulk properties of the metal. The quantities O, and of course F, are surface properties which can vary on a metal surface from one crystallographic plane to the other. Such variations are typically on the order of 0.1 eV but can be as high as 0.5 V. The measured work function , of a polycrystalline metal is an average of the d> values on different crystallographic planes. 

Clean, polycrystalline metals expose mostly their most densely packed surface, because, to create this surface, the minimum number of bonds have to be broken (see Fig. 5.3). 

The metal surface is polycrystalline and has a rather complex profile. Because of different crystallite orientations at the surface, different crystaf faces are exposed, such as smooth fow-index faces and stepped high-index faces. Surface texture where a particufar kind of face is predominant can devefop in individual cases. Microcracks and various lattice defects (dislocations, etc.) will also emerge at the surface. 

Beeck at Shell Laboratories in Emeryville, USA, had in 1940 studied chemisorption and catalysis at polycrystalline and gas-induced (110) oriented porous nickel films with ethene hydrogenation found to be 10 times more active than at polycrystalline surfaces. It was one of the first experiments to establish the existence of structural specificity of metal surfaces in catalysis. Eley suggested that good agreement with experiment could be obtained for heats of chemisorption on metals by assuming that the bonds are covalent and that Pauling s equation is applicable to the process 2M + H2 -> 2M-H. 

It is particularly helpful that we can take the Cu-Ni system as an example of the use of successive deposition for preparing alloy films where a miscibility gap exists, and one component can diffuse readily, because this alloy system is also historically important in discussing catalysis by metals. The rate of migration of the copper atoms is much higher than that of the nickel atoms (there is a pronounced Kirkendall effect) and, with polycrystalline specimens, surface diffusion of copper over the nickel crystallites requires a lower activation energy than diffusion into the bulk of the crystallites. Hence, the following model was proposed for the location of the phases in Cu-Ni films (S3), prepared by annealing successively deposited layers at 200°C in vacuum, which was consistent with the experimental data on the work function. 

There is a wealth of information available on CO chemisorption over single-crystal and polycrystalline platinum surfaces under ultrahigh-vacuum conditions research efforts in this area have gained a significant momentum with the advent of various surface analysis techniques (e.g., 2-8). In contrast, CO chemisorption on supported platinum catalysts (e.g., 9, 10, 11) is less well understood, due primarily to the inapplicability of most surface-sensitive techniques and to the difficulties involved in characterizing supported metal surfaces. In particular, the effects of transport resistances on the rates of adsorption and desorption over supported catalysts have rarely been studied. 

Figure 2. Work function of polycrystalline Au electrode emersed from 0.1 M HC104 as a function of emersion potential. The work function of the clean metal surface was 5.2 eV (19). If the NHE absolute half-cell potential (with respect to s) is 4.45 V, the bottom line is equal to the solution inner potential, tfg. If it is 4.85 V, the upper curve through the points is equal to 0g. As always with WF, the Fermi level is taken as zero.

In most of the early studies 9> of H20(as) the vapor was condensed on metal surfaces in the temperature range 77 K diffraction data, supplemented by new experimental studies, convinced Olander and Rice that most deposits obtained at or above 77 K are likely contaminated with crystalline ice. They established conditions for the deposition of pure H20(as) on a variety of substrates 10>. Briefly put, the temperature of the substrate should be low, preferably below 55 K, and the rate of deposition very small (a few mg/hour). There is evidence that H20(as) can be deposited on a substrate at 77 K if the deposition is slow enough. The use of high deposition rates at 77 K leads to polycrystalline ice Ic mixed with H20(as). A sample of pure H20(as) is stable indefinitely long (at least several months) if maintained below 20 K. At about 135 K, with some variation from sample to sample, the amorphous solid transforms spontaneously and irreversibly to ice Ic. 

According to a proposed definition, the electron work function

Fermi level of the metal across a surface carrying no net charge, and to transfer it to infinity in a vacuum. The work function for polycrystalline metals cannot be precisely determined because it depends on the surface structure it is different for smooth and rough surfaces, and for different... 

An important consequence of the presence of the metal surface is the so-called infrared selection rule. If the metal is a good conductor the electric field parallel to the surface is screened out and hence it is only the p-component (normal to the surface) of the external field that is able to excite vibrational modes. In other words, it is only possible to excite a vibrational mode that has a nonvanishing component of its dynamical dipole moment normal to the surface. This has the important implication that one can obtain information by infrared spectroscopy about the orientation of a molecule and definitely decide if a mode has its dynamical dipole moment parallel with the surface (and hence is undetectable in the infrared spectra) or not. This strong polarization dependence must also be considered if one wishes to use Eq. (1) as an independent way of determining ft. It is necessary to put a polarizer in the incident beam and use optically passive components (which means polycrystalline windows and mirror optics) to avoid serious errors. With these precautions we have obtained pretty good agreement for the value of n determined from Eq. (1) and by independent means as will be discussed in section 3.2. 

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