Au surfaces

For static water absorption (ASTM D1815) (53), the weight of water absorbed duting 30 min, 2 h, or 24 h, with aU surfaces of the leather exposed to the water, is measured. 

Water resistance is an important factor in concrete and masonry constmetion for the safety, health, and comfort of buUding occupants (see Cement). Several texts on concrete constmetion describe the methods for obtaining water resistance (72—76). The term waterproof describes concrete and masonry that is completely impervious to water and its vapor, whether or not the water is under pressure. Waterproof constmetion involves the use of some type of barrier that covers aU surface pores or capUlaries. Water repeUent describes concrete or masonry that repels water without significantly reduced permeabUity to water vapor. In this discussion, concrete and masonry are used synonymously. 

Figure 5 Si backscattering yiaids (anguiar scans) for normal incidance on the Si (111) (7x7) surface (solid squares) and the Si (111) [Jz x Jz) R30°-Au surface (open circies). The curve is the expected yield from a bulk terminated Si (111) surface. The scattering geometry is shown in the inset.

More recently, Silva et a/.447,448 have found that the temperature coefficients of dEa /dT for a number of stepped Au surfaces do not fit into the above correlation, being much smaller than expected. These authors have used this observation to support their view of the hydrophilicity sequence the low 9 (rs0/97 on stepped surfaces occurs because steps randomize the orientation of water dipoles. Besides being against common concepts of reactivity in surface science and catalysis, this interpretation implies that stepped surfaces are less hydrophilic than flat surfaces. According to the plot in Fig. 25, an opposite explanation can be offered the small BEod0/dT of stepped surfaces is due to the strong chemisorption energy of water molecules on these surfaces. 

On smooth Au surfaces, the adsorbed oxygen, which is only possible in molecular form [5], is not selective to form PO [6]. Therefore partial oxidation is contributed by oxygen adsorbed on Ti02. However, there are no direct experimental evidence whether O2 is adsorbed dissociatively or non-dissociatively. It is generally accepted that O2 adsorbs on Ti02 in a molecular form [7] and is activated at the Au/Ti02 interface [1]. 

The fundamental issue of epitaxial growth on polycrystalline substrates has been addressed in a more refined manner in relation to the electrodeposition of CdSe on metals. Polycrystalline, lll -textured Au surfaces were shown [17] to promote the electrodeposition of coherent, epitaxial CdSe quantum dot films over areas micrometers in size, i.e., much larger than the polycrystalline Au grains, despite the numerous grain boundaries present in the substrate. The Au films (considered as... 

Ultrathin films of CdS ranging in coverage from 25 to 200 ML were grown also by the previous method on Au substrates (of non-specified nature) and were characterized by quantitative Raman resonance [41], It was found that the electronic structure of the films in this coverage regime corresponds to that of bulk CdS. It was concluded also that ECALE does not involve growth by random precipitation of CdS onto the Au surface the thin deposited layers of the material were contiguous. 

The preparation of immobilized CdTe nanoparticles in the 30-60 nm size range on a Te-modified polycrystalline Au surface was reported recently by a method comprising combination of photocathodic stripping and precipitation [100], Visible light irradiation of the Te-modified Au surface generated Te species in situ, followed by interfacial reaction with added Cd " ions in a Na2S04 electrolyte. The resultant CdTe compound deposited as nanosized particles uniformly dispersed on the Au substrate surface. 

There are at least three types of sites in Au/AljOj those on AI2O3, those at the AU-AI2OJ interface, and those on the Au surface. The data in Tables 1, 2 and 3 show that these sites contribute differently on different catalysts. 

Au surfaces yielded analogous results. Here the authors foimd gauche defects only in the second layer, while the first represented a flat lying chain [133]. 

Iodine and bromine adsorb onto Au(l 11) from sodium iodide or sodium bromide solutions under an applied surface potential with the surface structure formed being dependent on the applied potential [166]. The iodine adsorbate can also affect gold step edge mobility and diffusion of the Au surface. Upon deposition of a layer of disordered surface iodine atoms, the movement of gold atoms (assisted by the 2-dimensional iodine gas on the terrace) from step edges out onto terraces occurs. However, this diffusion occurs only at the step edge when an ordered adlayer is formed [167]. 

As well as the adsorption of halogen atoms or molecules, the adsorption of halide anions to gold surfaces has been extensively studied and a comprehensive review of the area has been published by Magnussen [168]. The degree of specific adsorption to gold surfaces increases in the order F < Cl < Br < 1 with only weakly specifically adsorbed. The presence of halide anions can also affect the electrodeposition of organic molecules such as pyridine on Au surfaces with chloride and bromide solutions suppressing the formation of ordered N-bonded pyridine layers [169]. 

Biener, M.M., Biener, J. and Friend, C.M. (2007) Sulfur-induced mobilization of Au surface atoms on Au(l 11) studied by realtime STM. Surface Science, 601, 1659-1667. 

Joo, S.-W. (2006) Characterization of self-assembled phenyl and benzylisothiocyanate thin films on Au surfaces. Surface and Interface Analysis, 38, 173-177. 

The second procedure is different from the previous one in several aspects. First, the metallic substrate employed is Au, which does not show a remarkable dissolution under the experimental conditions chosen, so that no faradaic processes are involved at either the substrate or the tip. Second, the tip is polarized negatively with respect to the surface. Third, the potential bias between the tip and the substrate must be extremely small (e.g., -2 mV) otherwise, no nanocavity formation is observed. Fourth, the potential of the substrate must be in a region where reconstruction of the Au(lll) surface occurs. Thus, when the bias potential is stepped from a significant positive value (typically, 200 mV) to a small negative value and kept there for a period of several seconds, individual pits of about 40 nm result, with a depth of two to four atomic layers. According to the authors, this nanostructuring procedure is initiated by an important electronic (but not mechanical) contact between tip and substrate. As a consequence of this interaction, and stimulated by an enhanced local reconstruction of the surface, some Au atoms are mobilized from the Au surface to the tip, where they are adhered. When the tip is pulled out of the surface, a pit with a mound beside it is left on the surface. The formation of the connecting neck between the tip and surface is similar to the TILMD technique described above but with a different hnal result a hole instead of a cluster on the surface (Chi et al., 2000). 

Klein et al. used the same model as Demming et al. [17], and calculated the FEE for an Ag tip near an Au surface [22], which is similar to the experimental conditions of Pettinger et al. [23]. They showed that Raman signals of molecules below or in the near field of the tip can be enhanced to practically measurable levels, and the FEE is larger for smaller tip-sample gaps, being localized below the tip, as shown in Figure 1.3. 

Alivisatos and coworkers reported on the realization of an electrode structure scaled down to the level of a single Au nanocluster [24]. They combined optical lithography and angle evaporation techniques (see previous discussion of SET-device fabrication) to define a narrow gap of a few nanometers between two Au leads on a Si substrate. The Au leads were functionalized with hexane-1,6-dithiol, which binds linearly to the Au surface. 5.8 nm Au nanoclusters were immobilized from solution between the leads via the free dithiol end, which faces the solution. Slight current steps in the I U) characteristic at 77K were reflected by the resulting device (see Figure 8). By curve fitting to classical Coulomb blockade models, the resistances are 32 MQ and 2 G 2, respectively, and the junction... 

Li X, Gewirth AA. 2005. Oxygen electroreduction through a superoxide intermediate on Bi-modified Au surfaces. J Am Chem Soc 127 5252-5260. 

The mechanism of anodic oxidation of CO at polycrystalline Au remains uncertain. Several groups have reported that the voltammetry of Au in acidic electrolytes is straightforward, with a well-formed oxidation wave/peak [Stonehart, 1966 Gibbs et al., 1977 Kita et al., 1985 Sun et al., 1999]. There is, however, no voltammetric evidence for the adsorption of CO on the Au surface, and spectroscopic studies indicate only a weak interaction of CO with poly crystalline Au surfaces in acidic solutions [Kunimatsu et al., 1986 Cuesta et al., 2003]. Moreover, there is little evidence for the formation of oxidizing species at the potential where the oxidation process is observed. Certainly, the oxidation of CO occurs at a potential over 500 mV less positive than that where bulk Au oxide is formed, and, indeed, the formation of this oxide strongly... 

There have been even fewer studies of CO electro-oxidation on supported Au particles than on supported Pt. One may expect, however, on the basis of the studies on extended Au surfaces, that there will be a structure sensitivity to the CO electrooxidation reaction on supported Au particles. 

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