Low-index crystal planes

Henero E, Franaszczuk K, Wieckowski A. 1994. Electrochemistry of methanol at low index crystal planes of platinum An integrated voltammetric and chronoamperometric study. J Phys Chem 98 5074-5083. 

E. Herrero, K. Frenasczuk, and A. Wieckowski, J. Phys. Chem. 8 5074 (1994). Methanol eoxidation at low-index crystal planes. 

Certain materials, most notably semiconductors, can be mechanically cleaved along a low-index crystal plane in situ in a UHV chamber to produce an ordered surface without contamination. This is done using a sharp blade to slice the sample along its preferred cleavage direction. For example. Si cleaves along the (111) plane, while III-V semiconductors cleave along the (110) plane. Note that the atomic structure of a cleaved surface is not necessarily the same as that of the same crystal face following treatment by IBA. 

Stable morphologies where the free energy is minimized by the exposure of low-index crystal planes that exhibit closest atomic packing. 

One of the strengths of the approach employed here is that we have freedom over the choice of the transition metal for the tip and also the structure of the tip employed. Usually we use Pt and W tips and represent the tip apex as a pyramid-like cluster epitaxed on a substrate that is orientated along some low Miller index crystal plane (for example, 111, 110, or 100 surface planes). Generally we find that the structure of the tip has quite a big impact on the images obtained. Sharp tips, such as those constructed on Pt 100, Pt 111 or W 100 surfaces tend to yield higher resolution images than those obtained with more blunt tips (for example the 111 surface of (bcc) W as shown in Scheme II). However,... 

Figure 5,1,3 Atomic arrangements of the low-index surface planes of an FCC crystal. (Adapted from E. Masel, Principles of Adsorption and Reaction on Solid Surfaces, Wiley, New York, copyright 1996, p. 38, by permission of John Wiley Sons, Inc.)...

Single crystal surfaces are associated with planes in the unit cells pictured in Figure 5.1.2 and are denoted by indices related to the unit cell parameters. Several examples of various low-index surface planes are shown in Figure 5.1,3 for the face-centered cubic structure. 

There is a long history of calculations of adsorption potentials for simple gases adsorbed on the exposed low index Miller planes of ionic crystals, especially alkali halides (see the review [26] and references therein). The total interaction potential energy between an adsorbed molecule and the surface of a solid is generally expressed as a sum of dispersion, repulsion, induction, and electrostatic contributions (see, e.g.. Ref. [27]) ... 

Since the catalytic activity takes place not only on the three-component system Cu/ZnO/AljOj, but already on pure ZnO, the first step in a thorough analysis of the reaction mechanism is to investigate how the reactants, intermediates and products adsorb on ZnO. However, since ZnO catalysts are mostly prepared in wet chemical processes, it is difficult to find out whether the active sites are Zn or 0 atoms on regular low-index surface planes, edges or corners of ZnO micro crystals, defects such as O vacancies, or even more complicated species. Of course, the simplest possibility is to study processes at regular ZnO surface planes, as is always done in the surface science approach to heterogeneous catalysis. 

ZnO surfaces are more complex than those of the rock-salt type oxides Uke MgO and NiO. ZnO crystalhzes in the wurtzite structure in which each Zn cation is tetrahedrally coordinated to four O anions and vice versa [105]. This crystal structure has no inversion center. The most important low-index surface planes are two polar planes, the Zn-terminated ZnO(OOOl) and 0-termi-nated ZnO(OOO-l) plane, and two neutral planes, ZnO(lO-lO) and ZnO(l 1-20). According to Nosker et al. [106] and Tasker [107], the two polar surfaces are thermodynamically unstable, however, they can be easily prepared and characterized experimentally, and do even show rather regular (1x1) LEED patterns [108]. This indicates that they are not stabilized by major reconstructions or other modifications. Therefore, it was believed for a long time that both polar surfaces exist in an unreconstructed bulk-Hke trimcation. Several contradicting proposals have been made to explain how the stability of the polar un-... 

Infrared spectroscopy is frequently applied to investigate CO adsorption on electrodes, because CO is important as an intermediate and surface poison in many electrocatalytic reactions and the C-O stretching vibrational modes of the adlayer are sensitive to the chemical environment at the metal/solution interface. Infrared spectra of CO adsorbed on low-index surface planes of Pt single-crystal electrodes have become a benchmark for use in understanding the behavior of CO on other surfaces. Related approaches have been extended to bulk single-crystal metal electrodes that include Pd [66, 67], Ir [68-71], Rh [13, 70], Ru [72-74], Ni [75, 76] and Au [77]. 

Most fiindamental surface science investigations employ single-crystal samples cut along a low-index plane. The single-crystal surface is prepared to be nearly atomically flat. The surface may also be modified in vacuum. For example, it may be exposed to a gas that adsorbs (sticks) to the surface, or a film can be grown onto a sample by evaporation of material. In addition to single-crystal surfaces, many researchers have investigated vicinal, i.e. stepped, surfaces as well as the surfaces of polycrystalline and disordered materials. 

Beebe T P, Goodman D W, Kay B D and Yates J T Jr 1987 Kinetics of the activated dissociation adsorption of methane on low index planes of nickel single crystal surfaces J. Chem. Phys. 87 2305... 

Since oxidation of methanol is an electrocatalytic reaction with different adsorption steps, interactions of the adsorbed species with the metallic surface are important. Using platinum single-crystal electrodes, it has been proven that the electrooxidation of methanol is a surface-sensitive reaction. The initial activity of the Pt(llO) plane is much higher than that of the other low-index planes, but the poisoning phenomenon is so rapid that it causes a fast decrease in the current densities. The... 

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