Crystal-face specificity

It is clear from Eq. (27) that owing to the crystal face specificity of 0, Eaao is expected to vary with the crystallographic orientation as well. Moreover, since the interfacial term X results from interfacial molecular interactions, it must be face-specific also. For a well-defined metal surface, Eq. (27) becomes... 

Equation (32) suffers from the same shortcomings as Eq. (27). In particular, d/dT must be known independently for the same metal sample as the one used as an electrode. Moreover, in view of the crystal-face specificity of ff=o, its temperature coefficient is also expected to depend on the crystallographic orientation. Being a differential quantity, dEa=JdT is an even more delicate experimental quantity than Eaa0 itself. 

For the same reasons, data on single-crystal faces for metals such as Zn, Sb, Bi, Sn, and Cd have not been plotted in Fig. 15. In order to indicate the probable position of d-metal surfaces, the line described by Eq. (64) has also been drawn in Fig. 15. It is interesting that all the points for sd-metals fall between the sp- and the d-metal groups. The crystal face specificities of Eas0 for Sb and Bi are complicated by their semimetallic nature. In any case, no data on 0 exist for a series of faces of these elements (only electrochemical work functions are available).28,864... 

TFe data of Popov et alm for Ag contradict the above sequence. They found that pentanol adsorbed more strongly on Ag(100) than on Ag(l 11). Similarly, Cd(0001) adsorbs less strongly than pc-Cd.661 The data for Sb and Bi are to some extent contradictory since the trend is broadly correct but with scatter, which is attributed to the crystal face specificity of space-charge effects.153 For instance, adsorption of cyclohexanol on Bi conforms to the sequence (011) > (101) > (211) > (001) >(111), while the capacitance at a - Ovaries in the sequence (001) > (011) > (211) > (101) > (111). Thus only the faces (001), (211), and (111) are in the expected order. Surprisingly, the Cd data of Lust etal. show similarities with those of Naneva etal.,212 although capacitances disagree. Thus the order of cyclohexanol adsorbability is (1010) > (0001) while the capacitance varies in the order (1010) > (1120) > (0001), i.e., the other way round. In these cases one might wonder whether the G(M-B) term is really independent of face. 

Almost all that is known about the crystal face specificity of double-layer parameters has been obtained from studies with metal single-crystal faces in aqueous solutions. Studies in nonaqueous solvents would be welcome to obtain a better understanding of the influence of the crystallographic structure of metal surfaces on the orientation of solvent molecules at the interface in relation to their molecular properties. 

Crystal face specificity of the potential of zero charge, 21... 

Potential of zero charge cont.) contribution of the solvent, 158 Conway and Colledan, and the determination of, 34 on copper, and aqueous solution, 89 crystal phase and, 44 crystal face specificity of, 21 and the crystal surface specificity, 152 DeLevie, on the effect of the density of broken bonds on, 75 dependence upon crystal phase, 154 dependence upon time of measurement, 150,151... 

The reasons for the crystallite size effect are not known. There are several possibilities. If the selective oxidation reaction is crystal face specific, then the size effect is due to the fact that different proportions of various crystal faces are present on crystallites of different sizes. It is known that small crystallites supported on silica are more difficult to reduce than large crystallites. This different reducibility can contribute to the size effect. It is possible that the charge transfer ability of small crystallites is different. Since charge transfer is involved in the activation of gaseous oxygen which is active in degradation of surface intermediates, this could also be a contribution. It is clear that further work is needed to distinguish the possibilities. 

Interesting information exists about the crystal-face specificity/reactivity [75]. In ethene hydrogenation the (001) face of Ni is inactive, because this face, under the standard reaction conditions applied, is completely covered by carbonaceous deposits. The (111) and (Oil) faces are both active and differ by less than a factor of 2 in their activity. The data just mentioned are just another example of a frequently encountered phenomenon the crystal face specificity or the particle size sensitivity of a reaction is induced by a side reaction and is not caused by the reaction in question [76]. 

Crystal face specific determination of lEP by means of SFM scanning force microscope was discussed by Eggleston and Jordan [87]. When tip and sample are made of the same material the force at contract shows a minimum at lEP. Preliminary results for silica and hematite showed that this minimum was rather broad, but the method is promising. One shortcoming in this method is a rather subjective definition where is the tip-sample contact on the force-distance curves. 

Eggleston, C.M. and Jordan, G., A new approach to pH of point of zero charge measiuement Crystal-face specificity by scanning force microscopy (SEM), Geochim. Cosmochim. Acta. 62. 1919, 1998. 

A number of theories have been put forth to explain the mechanism of polytype formation (30—36), such as the generation of steps by screw dislocations on single-crystal surfaces that could account for the large number of polytypes formed (30,35,36). The growth of crystals via the vapor phase is beheved to occur by surface nucleation and ledge movement by face specific reactions (37). The soHd-state transformation from one polytype to another is beheved to occur by a layer-displacement mechanism (38) caused by nucleation and expansion of stacking faults in close-packed double layers of Si and C. 

While from a structural point of view metal/solution and metal/vac-uum interfaces are qualitatively comparable even if quantitatively dissimilar, in the presence of ionic adsorbates the comparability is more difficult and is possible only if specific conditions are met.33 This is sketched in Fig. 7. A UHV metal surface with ions adsorbed on it is electrically neutral because of a counter-charge on the metal phase. These conditions cannot be compared with the condition of a = 0 in an electrochemical cell, but with the conditions in which the adsorbed charge is balanced by an equal and opposite charge on the metal surface, i.e., the condition of zero diffuse-layer charge. This is a further complication in comparing electrochemical and UHV conditions and has been pointed out in the case of Br adsorption on Ag single-crystal faces.88... 

The idea in these papers67,223,224 was to identify the potential of the capacitance minimum in dilute electrolyte solutions with the actual value of Ea=o (i.e., <7ge0m( min) = Ofor the whole surface) and to obtain the value of R as the inverse slope of the Parsons-Zobel plot at min.72 Extrapolation of Cwom vs- to Cgg0m = 0 provides the inner-layer capacitance in the / C geom, and not C ea as assumed in several papers.67,68,223,224 In the absence of ion-specific adsorption and for ideally smooth surfaces, these plots are expected to be linear with unit slope. However, data for Hg and single-crystal face electrodes have shown that the test is somewhat more complicated.63,74,219,247-249 More specifically,247,248 PZ plots for Hg/... 

The adsorption of aliphatic alcohols, which adsorb on metals with the hydrocarbon tail facing the electrode surface, shows different patterns on real Ag crystal faces440,441 with respect to quasi-perfect single-crystal face electrodes.442-444 This specific point will be discussed in detail in Section III. 

The effect of the addition of various surface-active organic compounds (cyclohexanol, camphor) to an aqueous solution of Na2S04 in contact with Bi single-crystal faces has been studied by Raud etal 9 in using ellipsometry. SQ - was not specifically adsorbed, but at E> -0.5 V (SCE), slight oxidation of the Bi faces was possible. 

While the pzc of Hg in F solution has not changed by more than 1 mV for over 70 years, marginal variations are visible for Ga, Tl, In, Cd, Bi, Sn, and Sb that are related to electrolyte effects (weak specific adsorption or disturbance of the adsorbed water layer, as for Ga).847 Important variations can be seen, on the other hand, for polycrystalline Ag, Zn, Ni, Fe, and Cu. For all these metals a drop of the pzc to much more negative values has been recorded this is evidently related to an improvement in the preparation of the surface with more effective elimination of surface oxides. All these metals, with the exception of Ag, are naturally sensitive to atmospheric oxygen. Values of pzc for single-crystal faces first appeared in a 1974 compilation,23 in particular for the three main faces of Ag and for Au (110). Values for a number of other metals were reported in 1986.25 However, for sd-metals, an exhaustive, specific compilation of available experimental data was given by Hamelin etal. in 1983.24... 

Nucleation The formation of sohd crystalhne nuclei on a foreign substrate is basically subject to the same laws as the formation of noncrystalhne nuclei. Specific features found in the case of crystalline nuclei are (1) considerably higher ESE values (2) a faceted rather than spherical shape and (3) anisotropic properties (i.e., different ESE values o, for different crystal faces /) ... 

It has also to be remembered that the band model is a theory of the bulk properties of the metal (magnetism, electrical conductivity, specific heat, etc.), whereas chemisorption and catalysis depend upon the formation of bonds between surface metal atoms and the adsorbed species. Hence, modern theories of chemisorption have tended to concentrate on the formation of bonds with localized orbitals on surface metal atoms. Recently, the directional properties of the orbitals emerging at the surface, as discussed by Dowden (102) and Bond (103) on the basis of the Good-enough model, have been used to interpret the chemisorption behavior of different crystal faces (104, 105). A more elaborate theoretical treatment of the chemisorption process by Grimley (106) envisages the formation of a surface compound with localized metal orbitals, and in this case a weak interaction is allowed with the electrons in the metal. 

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