Monatomic steps

Figure Bl.19.6. Constant current 50 mn x 50 mn image of a Cu(l 11) surface held at 4 K. Tliree monatomic steps and numerous point defects are visible. Spatial oscillations (electronic standing waves) with a...

Figure Bl.19.13. (a) Tliree STM images of a Pt(l 11) surface covered witli hydrocarbon species generated by exposure to propene. Images taken in constant-height mode. (A) after adsorption at room temperature. The propylidyne (=C-CH2-CH2) species that fomied was too mobile on the surface to be visible. The surface looks similar to that of the clean surface. Terraces ( 10 mn wide) and monatomic steps are the only visible features. (B) After heating the adsorbed propylidyne to 550 K, clusters fonn by polymerization of the C H...

Figure C2.7.6. STM images of an Ru(OOOl) surface after dissociative adsorjDtion of NO at 315 K. (A) Image (38 nmx33 nm) showing two terraces separated by a monatomic step (black stripe). (B) Close-up (6 nmx4 mn) showing an O island and individual N atoms. Individual O atoms are imaged as dashes (arrow) [9]...

Smith SPE, Ahruna HD. 1999a. Structural effects on the oxidation of HCOOH by bismuth modified Pt(lll) electrodes with (110) monatomic steps. J Electroanal Chem 467 43-49. 

Developments in electron microscopy also promise to revolutionize other studies of the surfaces of solids in general and/of catalysts in particular. Previously, monatomic steps and other topographical features at the exterior surfaces of solids were best investigated by the powerful but cumbersome, and destructive technique of gold-decoration - see refs 69 and 70 for studies of alkali halide and layered sulphides, respectively. 

But darkfield conventional transmission electron microscopy can now reveal monatomic steps directly, as the micrograph in Fig. 12 shows (71). Using this kind of approach it should be possible to ascertain quantitatively the extent of the interaction between a catalyst and its underlying support. 

Figure 12. A dark-field transmission electron micrograph of a (lll)Ag platelet grown on single-crystal MoS2. The regions with different contrast differ in thickness by one monatomic step from one another. The larger the number marked on each region, the thicker the crystal (3, 71).

Figure 21. STM images of Au( 111) in O.OSAf H2SO4 + 1 xiM CUSO4 at two diffeient potential regions, (a) Bare Au( 111), (b) and (c) Au( 111) at potentials in the middle of two underpotential deposition peaks, (c) Monatomic steps of the substrate. (From Ref. 67 with the permission of the Royal Chemical Society.)...

The interference between He particle waves scattered from adjacent terraces, separated by a monatomic step, provide detailed information about the step density and even about the actual distribution of terrace widths... 

Figure 3.13. Top Model of an ideal (100) surface of a face-centered crystal (fee) lattice. Center and bottom Model of a vicinal surface of an fee cut at 12° to the (100) plane a) with straight monatomic steps and (Z ) monatomic steps with kinks along the steps. (From Ref. 11, with permission from Pergamon Press.)...

Figure 3.16. Some simple defects found on a low-index crystal face 1, the perfect flat face, a terrace 2, an emerging screw dislocation 3, the intersection of an edge dislocation with the terrace 4, an impurity adsorbed atom 5, a monatomic step in the surface, a ledge 6, a vacancy in the ledge 7, a kink, a step in the ledge 8 an adatom of the same type as the bulk atoms 9, a vacancy in the terrace 10, an adatom on the terrace. (From Ref. 12, with permission from Oxford University Press.)...

In the results obtained from an analysis of surface diffusion between steps (Fleischmann and Thirsk, 1960 Damjanovic and Bockris, 1963), the model is a simple one. The properties of steps (e.g., their movement and eventual formation of spirals) will be discussed in Section 7.15. Of course, they are not really simple straight edges. Scanning tunneling microscopy has made it possible to obtain images of the steps. A real monatomic step is shown in Fig. 7.139 and is seen to be quite frazzled. 

Fig. 7.139. In situ STM image showing the frazzled appearance of a monatomic step on Aa(111) substrate. System Ag(111)/10 MCuS04 + 5x1(T2 M H2S04 at E= 60 mV vs. SCE and T = 298 K. (Reprinted from E. Budevski, G. Staikov, and W. J. Lorenz, Electrochemical Phase Formation and Growth, p. 22, copyright 1996John Wiley Sons. Reproduced by permission of John Wiley Sons, Ltd.

The kinetics by which UPD layers form are qualitatively the processes already discussed. There are the electron transfer kinetics from the metal substrate to the depositing ion and the surface diffusion of the adions formed to edge sites on terraces. Complications occur, however, for there is the adsorption of ions to take care of and that brings up questions of which isotherm to use (Section 6.8). Three kinds of UPD formations are shown in Fig. 7.146. Thus Fig. 7.146 (c) shows ID phase formation along a monatomic step in the terraces on the single ciystal Fig. 7.146 (b) shows 2D nucleation at a step, and Fig. 7.146 (a) shows 2D nucleation on an atomically flat plane. 

Finally, one has to distinguish between underpotential deposition of M on S and alloy formation. Alloys can be formed electrochemically (Brenner, 1942). Underpotential deposition is usually a monatomic step affair. With the alloy, the foreign atoms go on building up until they form part of the new substrate, the alloy. 

Fig. 1. Surface structure often found on low-index crystal faces. 1, A terrace perfectly flat crystal face. 2, An emerging screw dislocation. 3, The intersection of an edge dislocation with a terrace. 4, A ledge or monatomic step, 5. A kink a step in a ledge. 6, A vacancy in a ledge. 7, An adsorbed growth unit on a ledge.

Helium scattering has been reported also from a Cu(lOO) crystal which was roughened electrochemieally. It was assumed that the surface consisted of randomly oriented 100 terraces separated by monatomic steps and the observed variation in scattered intensity as a function of incidence angle was attributed to interference between beams scattered from various terraces. It was concluded that atomic steps separated by terraces 50 unit meshes wide could be detected readily. 

ID monatomic steps generated by screw dislocations, intersection lines of grain boundaries, and/or stacking faults. 

Figure 2.6 In situ STM images of a freshly prepared Au(lll) substrate showing the initial thermally induced reconstruction rows (visible as stripes) for different surface areas [2.10]. System Au(lll)/ 10 M H2SO4 at = - 150 mV vs. SCE and T = 298 K. (a) top view of an atomically smooth surface, and (b) 3D representation of a face with a monatomic step. Reprinted by permission of Kluwer Academic Publishers.

Figure 2.10 In situ STM image showing the frazzled appearance of a monatomic step on Ag(lll) substrate 2.22). SystemAg(lll)/10 M CuS04 + 5 x lO M H2SO4at =60 mVvs. SCEandT=298 K. Reprinted from Surface Science Letters, Vol. 327, M. Dietterle, T. Will, D.M. Kolb, Step dynamics at the Ag(lll)-electrolyte interface, p. L495,1995, with kind permission of Elsevier Science.

Recently, monatomic steps, which look frazzled in STM images, were observed by several authors [2.19-2.22]. An example for a monatomic step on Ag(lll) is shown in Fig. 2.10. This phenomenon was attributed to a mobility of kink sites on step edges. 

A high-indexed surface zone with a high density of steps or growth sites (kink positions) would follow the same growth law as liquid metals, i.e., the Butler-Volmer relation. Vicinal faces, characterized by low-index surface zones separated by uniformly distributed monatomic steps show an intermediate behavior. 

A comparison between cyclic voltammograms using electrochemically grown and real silver single crystal substrates showed a significant influence of the density of monatomic steps on F at the adsorption peak Ai in Fig. 3.4 [3.93-3.95, 3.109]. Therefore, the assumption of an expanded superlattice structure Ag(lll)-(2 x 2) Pb at low For high AE (Table 3.1) is unrealistic. New experimental results have shown that a better approach is to assume a step decoration at low Fin the potential range of the adsorption peak Ai. 

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