Metal surface states

For correlating relative Eamo values with values in the UHV scale (0 values), two quantities must be known 0 and A0. Contact potential measurements at metal/solution interfaces can be measured.4 In that case the interfacial structure is exactly that in the electrochemical situation (bulk liquid phase, room temperature). However, 0 to convert E into 0 must be independently known. It may happen that the metal surface state is not exactly the same during the measurements of 0 and A0. 

Unfortunately, unbalanced half-cell potentials are often the case with biopotential electrodes. Primarily, this is due to different electrode metal surface states that occur through oxidation occurring with air exposure, tarnishing, metallurgical preparation, previous electrolyte exposure, or past history... 

Michaelides and co-workersi have shown theoretically that for monolayers and bilayers of water on Ru, the predominant overlap occurs between the 2pa type 3ai orbital of water with a d 2 state on Ru. In addition there is overlap between the lone-pair Ibi orbital on water and the Ru dz2 state. The overlap and mixing of the metal surface state with the Ibi orbital is significantly stronger. An electronic analysis shows that there is a charge depletion from the Ibi and Ru d 2 states with a charge accumulation on the lower lying dxz and d states. There is also a small charge increase between the O and the Rul l. 

Semiconductors have greatly varying densities of states and thus contributions from metal probe tips are less prominent. Metal surface state densities vary to a much smaller degree and are thus comparable to those of the tip states, making electronic spectroscopy of metals more complicated. To enable comparison between spectra obtained at various surface positions it is important that the tip structure, and thus density of states remains constant between measurements. Rearrangement of the tip apex can greatly affect the observed spectra and thus lead to spurious data interpretation. 

Consequently, a more thorough analysis of experimental and theoretical factors that can influence the steady-state kinetics but which are not evident from the stoichiometric reaction, and sometimes not from the detailed mechanism and the corresponding kinetic equations, has been carried out in the past decade. Experimentally, particular attention has been given to the waiting time and metal surface state effects theoretically. 

Since surface states with free-electron-like dispersion (Shockley type) have a low occupancy per surface unit cell and a low DOS at Ey, they are in general not considered to dominate the energetics of the surface, although - as discussed in Section 5.4.3 - situations exist where they can afiect the properties of the surface. On the contrary, metallic surface states derived from weakly dispersing bands (Tamm states) may have a high DOS at Ey and thus may influence the surface phase diagram considerably. As mentioned already in the case of quasi-2D states, transition metal surfaces are interesting in this respect, the question... 

Chemisorption occurs when the attractive potential well is large so that upon adsorption a strong chemical bond to a surface is fonued. Chemisorption involves changes to both the molecule and surface electronic states. For example, when oxygen adsorbs onto a metal surface, a partially ionic bond is created as charge transfers from the substrate to the oxygen atom. Other chemisorbed species interact in a more covalent maimer by sharing electrons, but this still involves perturbations to the electronic system. 

Hasselbrink E 1994 State-resolved probes of moleoular desorption dynamios induoed by short-lived eleotronio exoitations Laser Spectroscopy and Photochemistry at Metal Surfaces ed E H L Dai and W Ho (Hong Kong World Soientifio) p 685... 

Figure A3.10.23 Schematic diagram of molecular CO chemisorption on a metal surface. The model is based on a donor-acceptor scheme where the CO 5 a FIOMO donates charge to surface unoccupied states and the surface back-donates charge to the CO 2 71 LUMO [58].

Ultraviolet photoelectron spectroscopy (UPS) results have provided detailed infomiation about CO adsorption on many surfaces. Figure A3.10.24 shows UPS results for CO adsorption on Pd(l 10) [58] that are representative of molecular CO adsorption on platinum surfaces. The difference result in (c) between the clean surface and the CO-covered surface shows a strong negative feature just below the Femii level ( p), and two positive features at 8 and 11 eV below E. The negative feature is due to suppression of emission from the metal d states as a result of an anti-resonance phenomenon. The positive features can be attributed to the 4a molecular orbital of CO and the overlap of tire 5a and 1 k molecular orbitals. The observation of features due to CO molecular orbitals clearly indicates that CO molecularly adsorbs. The overlap of the 5a and 1 ti levels is caused by a stabilization of the 5 a molecular orbital as a consequence of fomiing the surface-CO chemisorption bond. 

The polarization dependence of the photon absorbance in metal surface systems also brings about the so-called surface selection rule, which states that only vibrational modes with dynamic moments having components perpendicular to the surface plane can be detected by RAIRS [22, 23 and 24]. This rule may in some instances limit the usefidness of the reflection tecluiique for adsorbate identification because of the reduction in the number of modes visible in the IR spectra, but more often becomes an advantage thanks to the simplification of the data. Furthenuore, the relative intensities of different vibrational modes can be used to estimate the orientation of the surface moieties. This has been particularly useful in the study of self-... 

Our intention is to give a brief survey of advanced theoretical methods used to detennine the electronic and geometric stmcture of solids and surfaces. The electronic stmcture encompasses the energies and wavefunctions (and other properties derived from them) of the electronic states in solids, while the geometric stmcture refers to the equilibrium atomic positions. Quantities that can be derived from the electronic stmcture calculations include the electronic (electron energies, charge densities), vibrational (phonon spectra), stmctiiral (lattice constants, equilibrium stmctiires), mechanical (bulk moduli, elastic constants) and optical (absorption, transmission) properties of crystals. We will also report on teclmiques used to study solid surfaces, with particular examples drawn from chemisorption on transition metal surfaces. 

In the final section, we will survey the different theoretical approaches for the treatment of adsorbed molecules on surfaces, taking the chemisorption on transition metal surfaces, a particularly difficult to treat yet extremely relevant surface problem [1], as an example. Wliile solid state approaches such as DFT are often used, hybrid methods are also advantageous. Of particular importance in this area is the idea of embedding, where a small cluster of surface atoms around the adsorbate is treated with more care than the surroundmg region. The advantages and disadvantages of the approaches are discussed. 

Kolb D M and Franke C 1982 Surface states at the metal-electrolyte interface Appl. Phys A 49 379-87... 

Interface states played a key role in the development of transistors. The initial experiments at Bell Laboratories were on metal/insulator/semiconductor (MIS) stmctures in which the intent was to modulate the conductance of a germanium layer by applying a voltage to the metal plate. However, only - 10% of the induced charges were effective in charging the conductance (3). It was proposed (2) that the ineffective induced charges were trapped in surface states. Subsequent experiments on surface states led to the discovery of the point-contact transistor in 1948 (4). 

Catalysis by Metals. Metals are among the most important and widely used industrial catalysts (69,70). They offer activities for a wide variety of reactions (Table 1). Atoms at the surfaces of bulk metals have reactivities and catalytic properties different from those of metals in metal complexes because they have different ligand surroundings. The surrounding bulk stabilizes surface metal atoms in a coordinatively unsaturated state that allows bonding of reactants. Thus metal surfaces offer an advantage over metal complexes, in which there is only restricted stabilization of coordinative... 

Although in the dry state carbon tetrachloride may be stored indefinitely in contact with some metal surfaces, its decomposition upon contact with water or on heating in air makes it desirable, if not always necessary, to add a smaH quantity of stabHizer to the commercial product. A number of compounds have been claimed to be effective stabHizers for carbon tetrachloride, eg, alkyl cyanamides such as diethyl cyanamide (39), 0.34—1% diphenylamine (40), ethyl acetate to protect copper (41), up to 1% ethyl cyanide (42), fatty acid derivatives to protect aluminum (43), hexamethylenetetramine (44), resins and amines (45), thiocarbamide (46), and a ureide, ie, guanidine (47). 

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