Surfaces stoichiometry

Creighton J R 1994 Surface stoichiometry and the roie of adsorbates during GaAs atomic iayer epitaxy App/. Surf. Sc/. 82/83 171 -9... 

Although considerable study has been devoted to oxygen chemisorption (mainly on platinum) there is considerable ambiguity in the surface stoichiometry of the reaction. In some cases Pt20 is formed, in others PtO, the particular compound... 

Orts JM, Rodes A, Feliu JM. 1997. Irreversibly adsorbed As at full blockage on Pt(lll) electrodes—Surface stoichiometry. J Electroanal Chem 434 121 -127. 

Both vanadium and niobium metals form dihydrides only at high pressures(27), and numerous phases with hydrogen compositions less than one(28). Experiments were performed to saturate vanadium clusters with deuterium. Figure 4 is a plot of the number of deuteri urn molecules found in the products. The solid straight lines are for D V ratios of 1 and 2. The corresponding curved dashed lines include corrections for some bulk atoms(2c). The best fit to the data including only surface atoms indicate a stoichiometry of 1.5. It is likely that this high surface stoichiometry is an indication of bulk i ncorporation of deuterium. 

It is worthwhile to mention the ample use of screening final states models in understanding core levels as well as valence band spectra of the oxides. The two-hole models, for instance, which have been described here, are certainly of relevance. Interpretational difference exists, for instance, on the attribution of the 10 eV valence band peak (encountered in other actinide dioxides as well), whether due to the non-screened 5f final state, or to a 2p-type characteristics of the ligand, or simply to surface stoichiometry effects. Although resonance experiments seem to exclude the first interpretation, it remains a question as to what extent a resonance behaviour other than expected within an atomic picture is exhibited by a 5 f contribution in the valence band region, and to what extent a possible d contribution may modify it. In fact, it has been shown that, for less localized states (as, e.g., the 3d states in transition metals) the resonant enhancement of the response is less pronounced than expected. 

J.R. Creighton, H.K. Moffat, and K.C. Baucom. Surface Stoichiometry, Structure, and Kinetics of GaAs MOCVD. Electrochem. Soc. Proc., 98-23 75-88,1998. 

After crystal illumination, spectrophotometric examination of the electrolyte by the pertitanate method (16) showed no dissolved titanium with a sensitivity on the order of 10 monolayers. It appears that no irreversible change in the surface stoichiometry of constituent elements accompanies the slow photogeneration of hydrogen on metal-free crystals in aqueous NaOH. Although a change in stoichiometry occurred in NaClOi, no such change occurs in other electrolytes which are also ineffective for hydrogen production. 

Since rinsing in water can change the stoichiometry of vacuum-prepared crystals, it is possible that this rinsing step may obscure reversible changes in surface stoichiometry which occur during immersion and/or illumination in diverse electrolytes. The use of volatile electrolytes or gas-phase reactants is thus desirable. 

If one could disregard the complicated influence of poisons on mass transfer processes, it would be possible to state in a first approximation that catalyst activity for a selected reaction is a monotonic function of the surface area occupied by the active component. The problem that arises is the measurement of the catalytic surface area in the presence of a support material. In the case of Pt such a measurement is relatively simple, done by hydrogen chemisorption (56, 57) or titration (55), although even in this case there are uncertainties associated with surface stoichiometry (59, 60). These problems become more complicated when Pd, or other noble metals are incorporated at the same time, and still more so, when the catalysts have been contaminated (61). 

The preparation or etching of compound semiconductors is more complex due to the potential of altering the surface stoichiometry. Shiota et al. (36) used AES to show that the final As/Ga at the surface of GaAs was very dependent on the chemical activity of wet chemical etchants. Bertrand was able to follow the changes in the chemical bonding of Ga and As on p-type GaAs etched in HC1 or Br in methanol and relate this to Schottky barrier heights of similarly prepared surfaces with Pb contacts (37). 

A practical problem is that the sputtered chemical complexes often exhibit only a transitory existence. Traditional mass spectrometric techniques provide information on gross removal but little understanding of the mechanism involved. Data requirements in this area include gross removal rates, nature of ejected species, and changes to surface stoichiometry. In Sect. 6.5.1., the influence of surface chemistry on the nature of the sputtered species, and in particular on secondary ion fractions will be discussed. The role of surface chemistry and surface modification procedures on impurity control will be elucidated there. 

Despite the utility of the physical methods described above, characterization of entities on the nanometer scale is still a problem. Well-mixed nanoparticles are not necessarily completely homogeneous, and one metal may preferentially segregate to the nanoparticle surface. Subtle differences in surface stoichiometries are presently extremely difficult to quantitatively evaluate with spectroscopy, even when the metal of interest has an intense surface plasmon. 

These results were interpreted in terms of a substantial surface enrichment in Cu, driven by Cu s lower heat of sublimation [23]. The reactivity of these catalysts for CO oxidation, and the clear spectroscopic evidence for surface Pt - CO species indicate that, at least for the heterogeneous systems, particle surface stoichiometries are very sensitive to metal-adsorbate interactions. Similar arguments were presented for the PtAu/silica system, in which monometallic Au particles severely sinter under dendrimer removal conditions. In this case, the retention of small bimetallic particles after activation was attributed to the strength of Pt-silica interactions, which effectively anchored the bimetallic nanoparticles to the support [24],... 

Polymer complex Reagent mole ratio monomer halogen Polymer composition (elemental analysis) Conductivity cr(S cm1) Deconvoluted halogen spectra" XPS surface stoichiometry N halogen ratio Nls spectra components (B.E. > 401 eV)... 

Further problems can arise because of uncertainties concerning the stoichiometry of the adsorption reaction. For most metals it is assumed that the surface stoichiometry with H2 is H/M = 1. However, there is evidence especially for very small metal particles (of the order of 1 -5 nm) that the stoichiometry can exceed H/M = 1. For quantitative measurements of surface area it is necessary to establish the chemisorption stoichiometry and structure. In practice it is usually possible to achieve approximate estimate of the surface area by some other independent method (for example, from particle size analysis by X-ray line broadening or by TEM). In the case of CO, the CO/M ratio is generally taken as 1.0, but the true value may depend on the particle size and on the particle morphology. With N2O the N2O/M ratio at monolayer coverage is usually assumed to be 0.5, but once again there is no certainty about the validity of this particular assumption. 

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