Step density

What gives rise to streaks in a RHEED pattern from a real surface For integral-order beams, die explanation is atomic steps. Atomic steps will be present on nearly all crystalline surfaces. At the very least a step density sufficient to account for any misorientation of the sample from perfeedy flat must be included. Diffraction is sensitive to atomic steps. They will show up in the RHEED pattern as streaking or as splitdng of the diffracted beam at certain diffraction conditions that depend on the path difference of a wave scattered from atomic planes displaced by an atomic step height. If the path difference is an odd muldple of A./2, the waves scattered... 

A further result of Sadler s 2D-simulation was a relation between the step density and growth rate on the one hand and the inclination of the surface with respect to the principal axes on the other. From this relation crystal shapes were derived which show considerable curvature. This result of an exact treatment stands in contrast to Frank s mean-field curvature expression which gives essentially flat profiles. We will return to the discussion of curved edges in Sect. 3.6.3. 

Figure 6.2a shows chronoamperometric transients for CO oxidation recorded on three different stepped electrodes for the same final potential. Clearly, the electrode with the higher step density is more active, as it oxidizes the CO adlayer in a shorter period of time. Figure 6.2b shows a fit of a transient obtained on a Pt(15, 15, 14) electrode (terrace 30 atoms wide) by both the mean field model [(6.5), solid line] and the N G model [(6.6), dashed line]. The mean field model gives a slightly better fit. More importantly, the mean field model gives a good fit of all transients on all electrodes. 

The potential of the stripping peak, and hence the activity of the electrode for CO oxidation, also depends on the platinum surface structure in general and on the step density in particular. Based on the chronoamperometry experiments described in Section 6.2.1.1, one would expect the stripping peak to shift to lower potential with increasing step density. That this is indeed the case is shown in Fig. 6.6. Again, this... 

Similar studies have been carried out with Pt(l 11) and stepped surfaces with (111) terraces [Angelucci et al., 2007a, b]. The voltammetric profiles of these surfaces agree qualitatively with those depicted in Fig. 6.9. For the stepped surfaces, the potentials Ex and 2 depend linearly on the step density for terraces wider than 5 atoms. This hnear dependence is a consequence of the dependence of the oxidation rate on the step density, as was observed in the chronoamperometric CO stripping experiments. In H2SO4... 

Tamowski DJ, Korzeniewski C. 1997. Effects of surface step density on the electrochemical oxidation of ethanol to acetic acid. J Phys Chem B 101 253. 

Voltammetry curves obtained by Vitanov et al. (8) on electro-chemically grown Ag(lll) surfaces with an ultra low step density... 

A similar effect was observed in our work and in the work of others (5), where voltammetry curves changed after extended cycling, particularly if the cathodic sweep was reversed before the full Pb deposition coverage. The observed "cathodic memory effect" may be due to the proposed structural transformation phenomenon and subsequent step density growth, initially facilitated by a high step density on a UHV-prepared or chemically polished (6) Ag(lll) substrate. Post electrochemical LEED analysis on Ag(lll)-Pb(UPD) surfaces provided additional evidence of a step density increase during Pb underpotential deposition, which will be discussed later in this text. (See Figure 3.)... 

Hydrogen adsorption and oxidation of formic acid show a pronounced dependence on the structure of single crystal surfaces. The influence of the terrace and step orientation and step density is reflected in both reactions on step surfaces. The multiple states of hydrogen adsorption can be correlated with the nature of adsorption sites. 

In contrast to curve in Fig. 1, two small peaks on a flat portion of the curve at E >-0.025 V and E=-0.15 V have been observed by several authors (4-6). Our data to be shown in the next section strongly suggest that such peaks are due to a stepped surface with a small step density, while a well-ordered Pt(lll) surface should give a curve as in Fig. 1. 

Both the data on hydrogen adsorption and formic acid oxidation show pronounced structural sensitivity, thus confirming a paramount role of surface structure in electrocatalytic reactions. It can be concluded that each crystallographic orientation represents a distinct electrochemical (chemical) entity. The investigation of stepped surfaces seems to be necessary to reach an understanding of these systems on a molecular level. Hydrogen adsorption shows dependences on the terrace orientation, step orientation, and step density. All the... 

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... 

In Figs. 4S and 46 we summarize the results obtained by these authors for the Ni(l IS) and Ni(l 13) surface. Figure 45 shows the peak width (FWHM) of the specular He beam scattered from the Ni(115) surface along the (5, S, 2) azimuth as a function of the incident angle 0,. The oscillation characterizes the step density. The lineshapes of the specular He peak are shown in Fig. 46 in a log-log plot for Ni(113) along the (3, 3,2) direction with the temperature as parameter. The profiles suggest the expected power-law behavior and the variation of the power exponent 7 (i.e. the slope) with temperature is obvious. 

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