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     

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.     

Unenhanced raman spectroscopy of benzene adsorbed on single crystal silver surfaces: Evidence for surface selection rules

We have interpreted the unenhanced Raman spectra of perdeuterobenzene adsorbed in submonolayer quantities on the (111) and (110) surfaces of silver to     

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.     

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.

     

Scanning tunneling microscopic investigations of the adsorption and segregation of carbon and sulfur on nickel single crystal surfaces

The adsorption and segregation of carbon or sulfur on Ni single crystal surfaces have been investigated by scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). Different adsorbate(segregate)-induced surface modifications have been detected in dependence on the original Ni surface orientation and the kind of nonmetal atoms: i) Adsorption of carbon from ethylene on Ni(111) at 6.7×10^–4 Pa and 1000K leads to the epitaxial growth of a graphitic carbon monolayer which exhibits the structure of the hexagonal basal plane of graphite. However, as is found for highly oriented pyrolytic graphite (HOPG), only three of six carbon atoms of the (0001) graphite plane are imaged by STM. In contrast, on Ni(771) at 663 to 1000 K carbon islands have been formed but no graphite monolayer formation is detected. This behavior can be understood by considering the aspects that no large-area epitaxy between the graphite basal plane and the Ni(110) terraces exists and that the surface carbon activity was too low to initiate substrate restructuring. ii) Segregation of sulfur (from the Ni bulk containing 5 to 7 ppm S) on Ni(110) at 1043K and _s0.4 ML initiates the growth of sulfur islands which show a c(2×2)-S overlayer structure, whereas on Ni(111) at 823K and _s0.2 ML (average value) a reconstructed surface phase is forming which can be described as an adsorbed two-dimensional sulfide Ni₂S.     

Atomic beam scattering from single crystal surfaces

Application of atom-scattering to a variety of surface problems is expanding rapidly, owing in large part to the extreme surface- sensitivity of this probe. Helium is particularly useful because of its low mass and chemical inertness. Beams with velocity spreads of less than one percent and wavelength of the order of one Angstrom can be formed by nozzle expansion.

The scattered flux from a clean, well-ordered crystal surface contains elastic and inelastic, coherent and incoherent, components. The coherent elastic component (i.e., the specular and diffracted beams) contains information about the crystallographic structure of the outer- most atomic layer of the crystal and about the interaction potential between the crystal and the scattered particle. The latter manifests itself in the form of resonances between the incoming free-particle state, and the two-dimensional Bloch states bound in the potential well at the surface. Elastic scattering theory has reached the point where the resonance signatures in the various diffracted beams can be predicted accurately.

Crystallographic information resides in the diffracted beam intensities. Theoretical interpretation is less well advanced, though some progress has been made with “hard-wall” models. Experimental studies of reconstructed surfaces and chemisorbed overlayers appear very promising.

In inelastic scattering, energy resolution has been achieved by both time-of-flight and diffraction methods. High-resolution studies on alkali halide surfaces have led to experimental determination of Rayleighwave dispersion relations over the full Brillouin zone. Preliminary results have also been obtained on some metals.     

Layer-by-layer adsorption of water molecules on the surface of a silver iodide crystal

Computer simulations at the molecular level were used to analyze the mechanism of the nucleation of water condensate from the vapor phase on the surface of a silver iodide crystal at 260 K. The initial stage of the condensation process is the sequential growth of monolayers on the substrate surface without formation of a compact microdroplet. The dependence of the equilibrium work of formation of the condensate film on its thickness exhibits oscillations. The formation of layers close to the substrate surface involves the overcoming of a Gibbs energy barrier.     

Selective determination of silver in solution by adsorption on a piezoelectric quartz crystal

Electrodeposition of metal ions on the crystal is eliminated by using a specially constructed transistorized oscillator. When tartrate, citrate, EDTA or their mixtures are present, silver adsorption occurs. The frequency change is proportional to the silver concentration in the range 2 × 10^?7?1 × 10^?5 M after adsorption for 10 min from a 1 mM EDTA/3 mM tartrate solution. No significant interferences are caused by other metal ions. On the basis of cyclic voltammetric studies, it is suggested that silver is adsorbed as a silver (I) complex.     

Characterizing single crystal surfaces using high resolution electron diffraction

Characterization and controlled manipulation of surfaces is a crucial factor in modern processing of the technologically relevant Si(100) surface. Using spot profile analyzing low energy electron diffraction, the morphological changes from a single stepped vicinal Si(100) surface to a single-domain (2×1) reconstructed surface have been investigated in situ during Si deposition. The temperature range for formation of this kinetically-stabilized single-domain surface was found to be 400–500 °C. This single-domain surface could be preserved for further characterization and experiments after quenching to room temperature.     

Effect of cationization on adsorption of silver nanoparticles on cotton surfaces and its antibacterial activity

Cotton was cationized by exhaustion method using 3-chloro-2-hydroxypropyl trimethyl ammonium chloride (CHPTAC) as a cation-generating agent. Adsorption of silver nanoparticles on normal and cationized cotton was studied by exhaustion method at temperatures of 80°C and 100°C. Two exhaustion baths were used, containing nanosilver colloidal solutions stabilized by two different stabilizers and various concentrations of silver nanoparticles. Fourier-transform infrared (FT-IR) spectra of normal and cationized samples confirmed the existence of quaternary ammonium groups on cationized cellulose fibers. X-ray diffraction (XRD) patterns showed that crystallinity of the modified cellulose fibers was decreased. Scanning electron microscope (SEM) images revealed that the surface of the modified cotton was rougher than that of normal cotton. In addition, SEM images showed the presence of silver nanoparticles on the surface of treated fabric samples. The amount of silver particles adsorbed on the fabric samples was determined using inductively coupled plasma-optical emission spectrometer. Antibacterial tests were performed against Escherichia coli bacteria as an indication of antibacterial effect of samples. Cationized cotton samples adsorbed more silver nanoparticles and then had greater ability to inhibit bacteria.     

Spectroscopic and electrochemical studies of transition-metal tetrasulfonated phthalocyanines: Part VI. The adsorption of iron tetrasulfonated phthalocyanine on single crystal silver electrodes

The influence of the underpotential deposition of Pb, Tl and Bi on the oxidation of ammonia and on the reduction and oxidation processes of hydroxylamine on platinum has been studied in alkaline solutions. Pronounced inhibition effects caused by underpotential metal adsorbates have been detected for either the oxidation of ammonia or the reduction of hydroxylamine. On the other hand, the same metal adsorbates markedly increase the electrocatalytic activity of the Pt electrode for hydroxylamine oxidation. The observed negative and positive catalytic effects have been interpreted in terms of the electronic and adsorption properties of the Pt substrate, which are strongly modified by the underpotential metal adsorbates.     

Adsorption on well-defined solid surfaces: Bromide adsorption on a (110) face of silver

The adsorption of bromide has been studied on a (110) face of silver by means of mixed solutions with hexafluorophosphate as base electrolyte. From the differential capacity-potential curves, the adsorbed charge of bromide was determined as functions of the electrode charge and the potential. The components of the inner-layer capacity have been determined for the two peaks always observed on the (110) face in the presence of specific adsorption. For the more negative peak potential, the component at constant amount adsorbed confirms the essential contribution of the orientation of water dipoles to the existence of this peak. On the other hand, the component at constant electrode charge indicates a non0linear electrode charge dependence of the standard Gibbs energy of adsorption. For the less negative peak potential, the two components of the inner-layer capacity seem to show that the bromide adsorption is mainly responsible for the appearance of the second peak.Different structures of the layer of adsorbed ions are proposed for different coverages of the surface. A pseudohexagonal structure, equivalent to the c (4×2) structure, corresponds to the adsorbed charge determined at saturation.     

Recent advances in the use of curved single crystal surfaces

In surface science, research traditionally employs macroscopically flat surfaces of single crystals. Curved surfaces have been applied more sporadically, but their history stretches back for many decades. Realization of the potential benefits and practical applications in surface physics and surface chemistry research progressed slowly in the 20th century. In more recent decades, research employing partial cylinders and dome-shaped crystals have found renewed interest. Modern surface sensitive techniques are being employed allowing the inherent large range of surface structures to reveal new insights. We briefly review the history, describe several types of surfaces and the range of structures they contain, suggest a notation for common types of curved surfaces, and discuss recent studies in more detail. We mainly focus on metal samples. We close with a short outlook.     

Preexponential factors for hydrogen desorption from single crystal metal surfaces

Partition functions for adsorbed hydrogen atoms have been evaluated for Ni(100), Ni(111), Pd(111) and Pt(111) surfaces. These account for the relative order of magnituted of the preexponential factors for associative desorption, ν_d, including the abnormally low value for the Pt(111)/H system. Furthermore application of transition state theory yields numbers for ν_d which are in good agreement with experimental data.     

Orientation dependent oxygen exchange kinetics on single crystal SrTiO(3) surfaces

The perovskite SrTiO(3) is arguably one of the most important oxide systems in condensed matter research. In this study, we report measurement of the orientation dependence of oxygen exchange on SrTiO(3) single crystal surfaces by dynamic conductivity measurements under electrochemical perturbations. Activation energy for electrical conduction in the 923-1223 K range at an oxygen partial pressure of ～10(-11) Pa of (100), (111), and (110) single crystals was found to be 2.6 eV, 2.7 eV, and 3.1 eV, respectively. The equilibration kinetics show profound dependence on the surface orientation and are modelled using a heterogeneous relaxation process. All surfaces show similar cationic sub-lattice limited rate behavior with (111), (100), and (110) having the fastest, intermediate, and slowest rates, respectively. We discuss the orientation dependence and its relation to local atomic structure in light of previous experimental and theoretical studies.     

STM study of glycine on TiO2(110) single crystal surfaces

The adsorption of glycine (NH2CH2COOH) was examined by scanning tunneling microscopy (STM) on TiO2(110) surfaces at room temperature. A (2x1) ordered overlayer was observed on the TiO2(110)-(1x1) surface. The adsorption of acetic acid and propanoic acid was also investigated on this surface and their STM images were quite similar to that of glycine. Since acetate and propanoate are formed by dissociative adsorption of these acids on TiO2(110), it is proposed that glycine adsorbs in the same way to form a glycinate. The amino group in the glycinate adlayer structurally analogous to those formed from aliphatic carboxylic acids would be extended away from the surface and potentially free to participate in additional reactions. The underlying structure of the TiO2 surface is important in determining the structure of the glycinate adlayer; no ordering of these adsorbates was observed on the TiO2(110)-(1x2) surface.