Lead adsorption on silver single crystal surfaces

Twin-electrode thin layer experiments have been carried out in the system Ag(hkl)/Pb²⁺, ClO₄^? in order to determine independently the charge and Pb²⁺ coverage fluxes of the lead underpotential adsorption. The electrosorption valency was found to be equal to the charge number of the lead ion. Therefore, the sorption behaviour of the system can be described by the metal monolayer model. The peak potentials of the cyclic voltammograms as well as the saturation coverages are only slightly dependent on the orientation of the silver single crystal surfaces used. Hypothetical superlattice structures for the monolayer formation are discussed. The Kolb-Gerischer linearity rule is considered critically.     

粘胶基碳纤维表面结构的STM研究

本文建立了用具有原子级分辨能力的扫描隧道显微镜 (STM)研究粘胶基碳纤维 (RCF)表面结构的方法。在较大尺度的STM图像上 ,RCF表面显得很粗糙 ,“峰”和“谷”的特征非常明显。增大放大率时 ,发现了约10nm宽的条状结构 ,其排列与纤维轴成一定角度 (45°～ 90°)。首次获得了RCF原子级的STM图像 ,在原子级尺度上 ,其原子排列并不规则 ,相邻原子间距为 0 .14 2nm ,最近六元环中心的距离是 0 .2 5 3nm。与高定向降解石墨 (HOPG)的对比研究进一步表明RCF表面的碳网是变形的六元环结构     

Second harmonic generation studies of anionic adsorption on polycrystalline and single crystal silver surfaces

Adsorption of anions on single crystal and polycrystalline silver surfaces has been monitored with optical second harmonic generation. Potentials inves     

Scanning tunneling microscopy on clean and adsorbate covered metal surfaces

The use of scanning tunneling microscopy (STM) for atomic scale characterization of clean and adsorbate covered (single-crystalline) metal surfaces is discussed. Topographic images reveal details on their periodic structure and on the atomic arrangement in the surface layer, and in particular on surface defects. The observation and characterization of individual adsorbate species gives access to the local electronic structure of the adsorption complex and to details of the chemical bond between substrate and adsorbate. Atomic resolution imaging opens new perspectives for the investigation of various surface processes such as surface diffusion, thin film growth or surface reactions.     

Scanning tunneling microscopy study of DNA-chromophore motif on solid surfaces

We focus our studies on DNA-chromophore motif on surfaces using samples prepared by the synthetic methods described by Wang and Li in a recent publication (J. Am. Chem. Soc. 2003, 125, 5248-5249). Scanning tunneling microscope (STM) was used to investigate the DNA-chromophore hybrids adsorbed on Au(111) and highly oriented pyrolytic graphite (HOPG) surfaces at room temperature in air. Experiments found that the DNA-chromophore hybrid molecules easily formed multimolecule aggregations on gold surface. On HOPG surfaces, however, DNA-chromophore hybrids were usually adsorbed as single molecules. STM images further showed DNA-chromophore hybrids adsorbed on Au(111) surfaces existed in the form of single molecule, dimer, trimer, tetramer, etc. The occurrence of molecular aggregations indicates that molecular interactions are comparable or stronger than molecule-substrate interactions; such weak interactions control the geometrical sizes and topographical shapes of the self-assembled DNA-chromophore hybrids on surfaces.     

Oscillatory reactions on single crystal surfaces

Heterogeneous catalytic reactions exhibit under certain conditions kinetic oscillations which have been investigated both with polycrystalline materials and with single crystal surfaces as catalysts. The present paper reviews single-crystal experiments conducted under isothermal, low pressure conditions (p < 10^-3 mbar). Two different reaction systems have been investigated: catalytic CO oxidation on various Pt and Pd orientations and catalytic NO reduction on Pt(100) using CO, H₂, or NH₃ as the reducing agent. The different reaction systems exhibit a wide variety of interesting phenomena which are well-known in nonlinear dynamics, for example, such as spatiotemporal pattern formation, the existence of Turing structures and the appearance of deterministic chaos, and chemical turbulence. The mechanistic steps leading to the observed phenomena have been investigated and appropriate mathematical models have been formulated and analyzed using bifurcation theory. The driving force for the rate oscillations has been shown to result from structural changes of the substrate in the case of catalytic CO oxidation on Pt surfaces, subsurface oxygen formation in the case of catalytic CO oxidation on Pd surfaces, and in the chemical reaction network described by a vacancy model in the case of the NO reduction reactions.     

Reactions of sulfur dioxide on calcium carbonate single crystal and particle surfaces at the adsorbed water carbonate interface

Sulfur dioxide reactions with calcium carbonate interfaces at 296 K in the presence and absence of adsorbed water result in the formation of adsorbed sulfite and sulfate. The extent of reaction is significantly enhanced, approximately five- to ten-fold for particulate and single crystal CaCO(3) (calcite), respectively, in the presence of adsorbed water between 30 and 85% RH. Atomic force microscopy following the reaction shows that adsorbed water facilitates surface reactivity by enhancing the mobility of surface ions, giving rise to the formation of nanometer sized product crystallites approximately 1 nm in height. Simultaneous with the formation of these crystallites is pitting and etching of the underlying substrate, which occurs preferentially in the vicinity of monoatomic surface steps. In the absence of water, there is little pitting and no evidence for the formation of crystallites. X-Ray photoelectron core and valence band spectra confirm the presence of two sulfur adsorbed species, SO and SO, with nearly equal amounts of SO and SO in the absence of adsorbed water and approximately five times more SO relative to SO in the presence of adsorbed water. From these data, it is proposed that the nanometer-sized crystallites are composed primarily of CaSO(3).     

Features of sulfur hexafluoride adsorption on carbon adsorbents

The isotherms describing excess adsorption of SF₆ and N₆I₆ on carbon adsorbents with different pore structures were measured at pressures of 0.001—2.4 and 0.0001—0.1 MPa, respectively, and temperatures of 298—408 E. A linear dependence of Henry"s constant on temperature in the lnK—10³/O coordinates was found for all the samples. The specific surface areas of the samples determined by the BET method from the SF₆ adsorption are lower than those derived from benzene adsorption. The most pronounced difference was found for the grafitized carbon black. When SF₆ was adsorbed on supermicroporous carbon AC-71 and on microporous carbons PAC and CMS, a hysteresis was found, which, unlike that on mesoporous carbon adsorbents, is observed in the initial region of the equilibrium pressures.     

Gas chromatographic investigations of the adsorption of n-octane on the surface of sulfur

Summary From the adsorption isotherms of n-octane on the surface of sulfur at 29.8°C and 40.6°C some thermodynamic functions were determined. On the basis of these data the structure of the adsorbed layer of n-octane is postulated. The molecular interactions and adsorption mechanism are also discussed. It is stated that both quasi-liquid and quasi-sold layers may be formed on the surface of sulfur depending on the surface coverage with n-octane.     

Quantitative measurement of the coverage on single crystal surfaces

Summary Two methods for the quantitative determination of the surface coverage of adsorbates on single crystal surfaces are discussed. In the first case a quartz crystal microbalance is used to calibrate an Auger spectrometer. The frequency change of the quartz and the Auger signal of the adsorbate are measured simultaneously during adsorption. With the calibrated Auger spectrometer the absolute coverage of oxygen on Ni (110) and Ni (111) is determined. The second method is used for adsorbates which can be thermally desorbed. The desorption spectra of the sample are compared with the calibration spectrum. This test spectrum can be obtained by introducing a well defined amount of gas into the vacuum chamber. The gas introduced produces a pressure increase comparable to a flash desorption spectrum. With this method the saturation coverages of H₂-Ni (111), H₂-Ni (100) and H₂-W (100) are determined.

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Absolutbestimmung adsorbierter Gasmengen an Einkristalloberflächen

Zusammenfassung Für die quantitative Bestimmung der Oberflächenbedeckung an Einkristallen werden zwei Methoden diskutiert. Im ersten Fall wird eine Schwingquarzmikrowaage zur Eichung des Augerspektrometers verwendet. Auf den Schwingquarz wird ein Adsorbens aufgedampft und anschließend das Augersignal des Adsorbats und die Frequenzänderung des Schwingquarzes während der Gasadsorption gleichzeitig gemessen. Mit diesem geeichten Augersignal werden absolute Bedeckungsbestimmungen an den Systemen O₂-Ni (110) und O₂-Ni (111) durchgeführt. Die zweite Methode eignet sich für thermisch desorbierbare Adsorbate. Die Desorptionsspektren der Probe werden mit einem Eichspektrum verglichen. Dieses erhält man, indem eine definierte Gasmenge in den Rezipienten eingelassen und damit ein Desorptionsspektrum erzeugt wird. Mit dieser Methode werden die Sättigungsbedeckungen der Adsorptionssysteme H₂-Ni (111), H₂-Ni (100) und H₂-W (100) bestimmt.

     

The influence of carbon on the adsorption of CO on a Rh(100) single crystal

The influence of carbon on the adsorption of CO from a Rh(100) single crystal has been studied by a combination of experimental techniques: Temperature Programmed Desorption (TPD), Low Energy Electron Diffraction (LEED), and High Resolution Electron Energy Loss Spectroscopy (HREELS). These experimental techniques were combined with a computational approach using Density Functional Theory (DFT). Using this combination of techniques, we have shown that surface carbon greatly influences adsorbed CO and we have determined the exact magnitude of this interaction. Furthermore, we have demonstrated that carbon does not remain fully on the surface; at higher coverage it diffuses partially to subsurface positions. The presence of these subsurface species significantly influences the adsorbates on the surface.     

On the mechanism of surfactant adsorption on solid surfaces: free-energy investigations

The adsorption free-energy of surfactant on solid surfaces has been calculated by molecular dynamics (MD) simulation for a model surfactant/solvent system. The umbrella-sampling with the weight histogram analysis method (WHAM) was applied. The entropic and enthalpic contributions to the full potential of mean force (PMF) were obtained to evaluate the detailed thermodynamics of surfactant adsorption in solid/liquid interfaces. Although we observed that this surfactant adsorption process is driven mainly by a favorable enthalpy change, a highly unfavorable entropic contribution still existed. By decomposing the free energy (including its entropic and enthalpic components) into the solvent-induced contribution and the surfactant-wall term, the effect of surface and solvent on the adsorption free-energy has been distinguished. The contribution to the PMF from the surface effect is thermodynamically favorable, whereas the solvent term displays an obviously unfavorable component with a monotonic increase as the surfactant approaches to the surface. The impact of various interactions from the surfaces (both solvent-philic and solvent-phobic) and the solvent on the adsorption PMF of surfactant has been compared and discussed. Compared to the solvent-philic surface, the solvent-phobic surface generates more stable site for the surfactant adsorption. However, the full PMF profile for the solvent-phobic system shows a clear positive maximum value at the bulk-interface transition region, which leads to a considerable long-range free-energy barrier to the surfactant adsorption. These results have been analyzed in terms of the local interfacial structures. In summary, this comprehensive study is expected to reveal the microscopic interaction mechanisms determining the surfactant adsorption on solid surfaces.     

Scanning tunneling microscopy of template-stripped Au surfaces and highly ordered self-assembled monolayers

Template stripping of Au films in ultrahigh vacuum (UHV) produces atomically flat and pristine surfaces that serve as substrates for highly ordered self-assembled monolayer (SAM) formation. Atomic resolution scanning tunneling microscopy of template-stripped (TS) Au stripped in UHV confirms that the stripping process produces a flat, predominantly 111 textured, atomically clean surface. Octanethiol SAMs vapor deposited in situ onto UHV TS Au show a c(4 x 2) superlattice with (square root 3 x square root 3) R30 degrees basic molecular structure having an ordered domain size up to 100 nm wide. These UHV results validate the TS Au surface as a simple, clean and high-quality surface preparation method for SAMs deposited from both vapor phase and solution phase.     

Scanning tunneling microscopy and spectroscopy of wet-chemically prepared chlorinated Si111 surfaces

Chlorine-terminated Si(111) surfaces prepared through the wet-chemical treatment of H-terminated Si(111) surfaces with PCl5 (in chlorobenzene) were investigated using ultrahigh vacuum scanning tunneling microscopy (UHV cryo-STM) and tunneling spectroscopy. STM images, collected at 77 K, revealed an unreconstructed 1 x 1 structure for the chlorination layer, consistent with what has been observed for the gas phase chlorination of H-terminated Si(111). However, the wet-chemical chlorination is shown to generate etch pits in the Si(111) surface, with an increase in etch pit density correlating with increasing PCl5 exposure temperatures. These etch pits were assumed to stabilize the edge structure through the partial removal of the <112> step edges. Tunneling spectroscopy revealed a nonzero density of states at zero bias. This is in contrast to the cases of H-, methyl-, or ethyl-terminated Si(111), in which similar measurements have revealed the presence of a large conductance gap.     

FTIR-spectroscopy as a highly sensitive technique to study adsorption and desorption on ionic film and single crystal surfaces

Sample/spectrum relationships are investigated using both a low resolution rapid-scanning NIR monochromator and a Fourier transform instrument capable of high resolution and are evaluated in terms of whether or not the resolution of the instrument is sufficient for measuring the natural bandwidths corresponding to the sample. Based on the sample/spectrum relationship a criterion is developed which must be followed in order to apply either derivative spectroscopy or deconvolution to enhance the resolution of overlapped bands without generating spectral artifacts.