Surface electronic properties

Mayrhofer KJJ, Blizanac BB, Arenz M, Stamenkovic VR, Ross PN, Markovic NM. 2005b. The impact of geometric and surface electronic properties of Pt-catalysts on the particle size effect in electrocatalysis. J Phys Chem B 109 14433-14440. 

Indium Tin Oxide Composition and Surface Electronic Properties 492... 

This chapter summarizes recent experimental findings on transition metal carbides and nitrides and shows, in section 25.2, that measurable surface shifts do exist both in the metal and nonmetal levels. A few application examples are presented in section 25.3. These experimentally determined surface shifts may stimulate new theoretical efforts to explain their existence and lead to a better understanding of the surface electronic properties. To date, no ab initio calculations of surface shifts for transition metal nitrides and carbides, including final state relaxation effects, have... 

The electronic structure of a solid metal or semiconductor is described by the band theory that considers the possible energy states of delocalized electrons in the crystal lattice. An apparent difficulty for the application of band theory to solid state catalysis is that the theory describes the situation in an infinitely extended lattice whereas the catalytic process is located on an external crystal surface where the lattice ends. In attempting to develop a correlation between catalytic surface processes and the bulk electronic properties of catalysts as described by the band theory, the approach taken in the following pages will be to assume a correlation between bulk and surface electronic properties. For example, it is assumed that lack of electrons in the bulk results in empty orbitals in the surface conversely, excess electrons in the bulk should result in occupied orbitals in the surface (7). This principle gains strong support from the consistency of the description thus achieved. In the following, the principle will be applied to supported catalysts. 

All of the studies discussed above for silver have been done with an incident beam of 1064 nm. These studies have proven that the anisotropy in the nonlinear polarizability from the silver surface is not purely free-electron-like at these wavelengths, that the anisotropy can be correlated with surface symmetry, and that the SH response measured in situ is nearly identical to that measured in UHV. The issue of the sensitivity of the rotational anisotropy to surface electronic properties has been the topic of very recent work which has been conducted by variation of the incident wavelengths to where optical resonances in the bulk or surface electronic structure can be accessed. 

These questions lead on to further fundamental questions concerning the shapes and properties of small metal particles. For example, what is the stable shape for a small metal particle How is this affected by size, method of preparation, temperature, gaseous environment, precursor compound, support morphology, etc. Do small metal particles have different electronic properties from bulk metal Do surface electronic properties depend on particle size, and if so, do they vary in the same way as bulk electronic properties When, indeed, is a particle small enough to have unusual properties ... 

Bulk electronic properties are observed for particles containing about 150 atoms, corresponding to diameters in the range 1.5-2.0 nm. On the other hand, surface electronic properties typical of bulk metal are observed with much smaller particles. Only about 25 atoms are required in a particle for the adsorption to the be characteristic of the bulk metal. 

Surface electronic properties may be measured Large field of view, from 1A to 100 pm... 

Wolfram, T. (1981). Chemisorption and surface electronic properties of the d-band oxides. J. Vac. Sci. Technol. 18, 428-32. 

The catalytic activity of Pti c polymer-protected bimetallics has been found to vary strongly with composition [75]. As shown in Figure 11, such catalysts have been studied by Pt NMR. However, because the palladium NMR has not been observed, the average surface electronic properties can be determined only indirectly and tentatively. The Pt NMR in Figure 11 has shown that the interior of the alloy particles is bulklike. In the bulk alloys the f-LDOS on both Pt and Pd sites varies rapidly with composition around x = 0.8 [54]. It is supposed, but not proven, that on the surfaces of the alloy particles the f-LDOS changes strongly with composition as well and that this explains the variation in catalytic activity. 

Surface activity (a) strength and mode of adsorption (b) adsorption isotherms (c) adatom formation (d) substrate-catalyst interactions (e) surface diffusion (f) adsorbate spillover (g) bulk electronic properties (h) surface electronic properties. 

Together with the surface electronic properties listed in Table Tl, four integrated electronic properties are also included in the set of TAE descriptors total energy, electron population, volume, and surface. 

Tompkins (1978) concentrates on the fundamental and experimental aspects of the chemisorption of gases on metals. The book covers techniques for the preparation and maintenance of clean metal surfaces, the basic principles of the adsorption process, thermal accommodation and molecular beam scattering, desorption phenomena, adsorption isotherms, heats of chemisorption, thermodynamics of chemisorption, statistical thermodynamics of adsorption, electronic theory of metals, electronic theory of metal surfaces, perturbation of surface electronic properties by chemisorption, low energy electron diffraction (LEED), infra-red spectroscopy of chemisorbed molecules, field emmission microscopy, field ion microscopy, mobility of species, electron impact auger spectroscopy. X-ray and ultra-violet photoelectron spectroscopy, ion neutralization spectroscopy, electron energy loss spectroscopy, appearance potential spectroscopy, electronic properties of adsorbed layers. 

By tailoring the surface structure and composition of a nanocatalyst, one can in principle vary the surface electronic properties to ultimately design an appropriate catalyst. Consequently, it is necessary to understand their electronic structure if one is to understand the synergism of heterometallic catalysts. Since experimental information on the structural and dynamic properties of supported and unsupported metal clusters is limited, a computational/modelling study is a useful complement to experimental studies. 

Surfaces - an introduction/Structure of surfaces/Thermodynamics of surfaces/ Dynamics at surfaces/Electronic properties of surfaces/The surface chemical bond/ Catalysis by surfaces/Mechanical properties of surfaces... 

Oq is that block of matrix Eq.(2.228b) which contains only the atoms of the metal surface. It gives the surface electronic properties before chemisorption. 

The chemistry of a surface determines the surface electronic properties. The following chapter was therefore focused on the influence of adsorbates. It was found that hydrogen exhibit unusual adsorption characteristics demonstrating a lagoon-like appearance. The incorporation of hydrogen in Gd thin films leads to a plastic deformation resulting in surface modihcations which were identified by STM. The combination of ultraviolet photoelectron spectroscopy (UPS) and STM allowed to determine the reaction scheme of coadsorption processes, exemplarily presented for hydrogen covered Gd surfaces exposed to CO. 

It is important to keep in mind that the chemical behavior of a surface determines the surface electronic properties. Thus, variations, e.g. due to adsorbate atoms, have a significant influence. This aspect will be focused on as the next topic with the description of selected substrate layers which were exposed to different types of gaseous molecules. 

Proportions of atoms having unusually low CN will change relatively little as size is increased beyond about 2 nm (ca. 60 % dispersion). (2) Bulk electronic properties are not likely to be shown by particles having less than about 150 atoms (1.7 nm, 70 % dispersion) (3) Typical surface electronic properties are however probably shown by particles with only 25-30 atoms (90 % dispersion). (4) Unusual crystallographic structures are rare. (5) Since the heat released by chemisorption is enough to convert one structure into another, metastable stmctures are unlikely to survive under reaction conditions, and surface reconstruction may occur frequently. (6) A small metal particle may comprise a solid core and a semi-fluid surface layer. 

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