Surface structures, different

The main problem in the analysis of E vs. 0 plots is that the two quantities are usually measured independently on different samples. It may happen that the surface structure differs somewhat so that for the sample on which E is measured is different from that of the sample used in UHV experiments. This is especially the case with polycrystalline surfaces, whose structural reproducibility is occasional, but it is also the case with well-defined crystal faces if reconstruction phenomena are possible.60 The problem persists also in the absence ofreconstruction since the concentration and/or distribution of surface defects may be differ-... 

Approaching a potential from more active potentials at a certain scan rate will create a surface structure different from that created when approaching the potential from more noble potentials. The positive hysteresis shown in Fig. 7.20 is caused by the polarization to more noble potentials making the surface more passive. The negative hysteresis in Fig. 7.19 is caused by a decrease in passivity, often produced by the initiation of localized corrosion. This latter phenomenon is usually a reflection of a propensity for localized corrosion in the form of either pitting or crevice corrosion. From a practical standpoint, a positive hysteresis usually signifies that the alloy will be more resistant to localized corrosion than does a negative hysteresis. ... 

Above 81.5 K the C(2x 1) structure becomes the more stable. Two important points are, first, that a change from one surface structure to another can occur without any bulk phase change being required and, second, that the energy difference between dtemative surface structures may not be very large, and the free energy difference can be quite temperature-dependent. 

A catalyst may play an active role in a different sense. There are interesting temporal oscillations in the rate of the Pt-catalyzed oxidation of CO. Ertl and coworkers have related the effect to back-and-forth transitions between Pt surface structures [220] (note Fig. XVI-8). See also Ref. 221 and citations therein. More recently Ertl and co-workers have produced spiral as well as plane waves of surface reconstruction in this system [222] as well as reconstruction waves on the Pt tip of a field emission microscope as the reaction of H2 with O2 to form water occurred [223]. Theoretical simulations of these types of effects have been reviewed [224]. 

At a surface, not only can the atomic structure differ from the bulk, but electronic energy levels are present that do not exist in the bulk band structure. These are referred to as surface states . If the states are occupied, they can easily be measured with photoelectron spectroscopy (described in section A 1.7.5.1 and section Bl.25.2). If the states are unoccupied, a teclmique such as inverse photoemission or x-ray absorption is required [22, 23]. Also, note that STM has been used to measure surface states by monitoring the tunnelling current as a fiinction of the bias voltage [24] (see section BT20). This is sometimes called scamiing tuimelling spectroscopy (STS). 

Flowever, it is necessary to first discuss the meaning of diffraction , because this concept can be interpreted in several ways. After these fiindamental aspects are dealt with, we will take a statistical and historical view of the field. It will be seen that many different diffraction methods are available for surface structural detemiination. 

One fiirther method for obtaining surface sensitivity in diffraction relies on the presence of two-dimensional superlattices on the surface. As we shall see fiirtlrer below, these correspond to periodicities that are different from those present in the bulk material. As a result, additional diffracted beams occur (often called fractional-order beams), which are uniquely created by and therefore sensitive to this kind of surface structure. XRD, in particular, makes frequent use of this property [4]. Transmission electron diffraction (TED) also has used this property, in conjunction with ultrathin samples to minimize bulk contributions [9]. 

A large number of ordered surface structures can be produced experimentally on single-crystal surfaces, especially with adsorbates [H]. There are also many disordered surfaces. Ordering is driven by the interactions between atoms, ions or molecules in the surface region. These forces can be of various types covalent, ionic, van der Waals, etc and there can be a mix of such types of interaction, not only within a given bond, but also from bond to bond in the same surface. A surface could, for instance, consist of a bulk material with one type of internal bonding (say, ionic). It may be covered with an overlayer of molecules with a different type of intramolecular bonding (typically covalent) and the molecules may be held to the substrate by yet another fomi of bond (e.g., van der Waals). 

Different values of will result if the integral limits (i.e., band width) or modulation transfer function in the integral change. All surface characterization instruments have a band width and modulation transfer function. If rms roughness values for the same surface obtained using different instruments are to be compared, optimally the band widths and modulation transfer functions would be the same they should at least be known. In the case of isotropic surface structure, the spatial frequencies p and q are identical, and a single spatial frequency (/>) or spatial wavelength d= /p) is used to describe the lateral dimension of structure of the sample. 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

At this point we should also recall another application of the already mentioned Bernal model of amorphous surface. Namely, Cascarini de Torre and Bottani [106] have used it to generate a mesoporous amorphous carbonaceous surface, with the help of computer simulation and for further application to the computer simulation study of adsorption. They have added a new component to the usual Bernal model by introducing the possibility of the deletion of atoms, or rather groups of atoms, from the surface according to some rules. Depending on the particular choice of those rules, surfaces of different porosity and structure can be obtained. In particular, they have shown examples of mono- as well as pohdispersed porous surfaces... 

In the systems characterized by more complex surface structure, consisting of patches of different size and different magnitude of the boundary field, the properties of adsorption isotherms have been found to depend on the... 

Considering the changes of E upon fi for different values of the parameters, Villain has concluded that imperfections in the surface structure may have a very strong influence on the behavior of incommensurate phases and on the C-IC transition. The usual lowering of symmetry during the C-IC transition does not occur here and the nature of the C-IC transition may be quite different from that on the surface free of defects. 

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