Roughness induced surface acoustic resonances

The vibrational and electronic spectra of a semi-infinite crystal with a planar surface is modified in presence of surface inhomogeneities or roughness such as ridges or grooves, quantum wires or tips… Using a Green's function formalism, we present an exact numerical method for obtaining the variation of the density of states associated with the adsorption of a ridge on a flat surface or with a groove cut into an otherwise planar surface. This general method is applied to the determination of the acoustic resonances of shear horizontal polarization associated with such deterministic surface protuberances or indentations. The positions and widths of the peaks in the total or local densities of states give the frequencies and lifetimes of the resonances, which may be more or less pronounced features depending on the relative parameters of the substrate and ridge materials. We also investigate the modifications of these acoustic surface shape resonances due to the interaction between two such defects. This calculation can also be transposed to the study of electronic structure of a wire near a flat surface, in the framework of an effective mass model.     

智能表面

智能表面就是在基材表面自组装成单分子层或接枝(环境敏感性)线性高分子刷,通过环境刺激,使表面性能如亲水/憎水、酸性/碱性、导电/绝缘、粘结/排斥、吸附/脱附等发生(可逆)变化。根据环境刺激的类型可以分为以下几类:溶剂响应、温敏性、电场响应、pH响应型以及光敏性智能表面。本文对此五种类型的智能表面作一介绍。     

Atom-probe field ion microscopy

The atom probe field ion microscope (AP-FIM) is a combination of a field ion microscope and a time-of-flight mass spectrometer with a single ion detection sensivity. With the field ion microscope topology of a surface, surface reactions and surface modifications can be studied in atomic detail. By time-of-flight measurements surface layers and interface layers can be chemically analyzed atom by atom and atomic layer by atomic layer. Compositional variations according to surface or interface segregation, precipitations, or surface changes in corrosion or in electrochemical layer formation etc. can be studied quantitatively on a subnanometer scale. Some of our studies on related problems will be decribed briefly.     

Interaction between an ion-penetrable particle and a solid particle. 1. Electrical double-layer interaction

Expressions are derived for the force and potential energy of the electrical double layer interaction between two parallel plates of different nature, i. e., an ion-penetrable plate and an ion-impenetrable plate. The latter may have either constant surface potential or constant surface charge density. It is shown that when the ion-impenetrable plate has a constant surface potential, the interaction force may, under certain conditions, become attractive even if the surface potentials of the two plates at infinite separation are of the same sign. In contrast, when the ion-impenetrable plate has a constant surface charge density, the interaction force may, under certain conditions, become repulsive even if the two plates at infinite separation are of opposite sign. This means that an ion-penetrable plate shows a dual behavior. That is, under certain conditions, it behaves like a solid plate with constant surface potential or surface charge density, depending on whether it interacts with a solid plate having a constant surface potential or a constant surface charge density.     

Changes in surface stress of gold electrode during underpotential deposition of copper

The changes in surface stress of the evaporated gold electrode (mainly oriented to the (111) plane) during underpotential deposition (UPD) of copper in 0.1 M sulfuric acid medium or 0.1 M perchloric acid medium with and without sulfate or chloride were measured using a bending beam method. The surface stress maximum of gold electrode appeared during Cu-UPD. The co-adsorption of (bi)sulfate or chloride ions with copper atoms induced the compressive surface stress to promote the Cu-UPD. The factors influencing the surface stress or surface elastic strain were discussed in relation to the Cu-UPD structure. Dedicated to Professor Su-Il Pyun on the occasion of his 65th birthday.     

Surface anchored polymer: Role in adhesion and friction

We present an investigation of thin polymer layers formed by either strong adsorption or end grafting on a surface or an interface. Depending on the kind of surface attachment, different internal organisations of the chains are observed: either polydisperse loops for adsorbed layers, or almost monodisperse tails in the case of grafting. The molecular parameters of the layer (length and surface density of anchored chains) and the molecular organisation inside the layer govern the ability of the surface anchored chains to be swollen by a good solvent or to penetrate into a bulk polymer, either a melt or a cross-linked elastomer, three properties which have been characterised through neutrons reflectivity techniques. We then analyse how the ability of the surface anchored chains to penetrate into a bulk polymer, entangle with it, and then be deformed when this bulk polymer is mechanically solicitated, are key parameters which govern adhesion and friction properties.     

Enhanced adsorption energy of Au1 and O2 on the stoichiometric TiO2(110) surface by coadsorption with other molecules

During heterogeneous catalysis the surface is simultaneously covered by several adsorbed molecules. The manner in which the presence of one kind of molecule affects the adsorption of a molecule of another kind has been of interest for a long time. In most cases the presence of one adsorbate does not change substantially the binding energy of another adsorbate. The calculations presented here show that the stoichiometric rutile TiO(2)(110) surface, on which one of the compounds -OH, Au(3), Au(5), Au(7), Na, K, or Cs or two different gold strips was preadsorbed, behaves differently: the binding energy of Au(1) or O(2) to such a surface is much stronger than the binding to the clean stoichiometric TiO(2)(110) surface. Moreover, the binding energy of Au(1) or O(2) and the amount of charge they take from the surface when they adsorb are the same, regardless of which of the above species is preadsorbed. The preadsorbed species donate electrons to the conduction band of the oxide, and these electrons are used by Au(1) or O(2) to make stronger bonds with the surface. This suggests that adding an electron to the conduction band of the clean stoichiometric TiO(2)(110) slab used in the calculation will affect similarly the adsorption energy of Au(1) or O(2). Our calculations show that it does. We have also studied how the preadsorption of Au(4) or Au(6) affects the binding of Au(1) or O(2) to the surface. These two gold clusters do not donate electrons to the surface when they bind to it and therefore should not influence substantially the binding energy of Au(1) or O(2) to the surface. However, adsorbing O(2) or Au(1) on the surface forces the clusters to change their structure into that of isomers that donate charge to the oxide. This charge is used by Au(1) or O(2) to bind to the surface and the energy of this bond exceeds the isomerization energy. As a result the surface with the isomerized cluster is the lowest energy state of the system. We believe that these results can be generalized as follows. The molecules that we coadsorbed with Au(1) or O(2) donate electrons to the oxide and are Lewis bases. By giving the surface high energy electrons, they turn it into a Lewis base and this increases its ability to bind strong Lewis acids such as Au(1) and O(2). We speculate that this kind of interaction is general and may be observed for other oxides and for other coadsorbed Lewis base-Lewis acid pairs.     

Chemical composition and structure of the surface layers-products of interaction between steel 3 and hostile aqueous environment: The effect of the environment

The composition and structure of layers formed by corrosion products at the open-circuit and passive potentials on the surface of steel 3 in aqueous media containing sulfate ions, borate ions, or their mixtures are studied by X-ray photoelectron spectroscopy and secondary ion mass spectrometry. The composition and structure of both the phase surface layers and the adsorption surface layers are found to depend on the solution nature. The composition and structure of a corrosion product layer; the composition of the double layer, in particular, the surface charge and potential; and the anion role as a corrosion inhibitor or activator depend on the way the anion coordinates with the corroding-metal surface. The coordination may be strong, in an inner coordination sphere (borate ions), or less strong, in an outer coordination sphere (sulfate ions) of a cation on the surface.     

Adsorption of cobalt ions on the "electrolytic solution/gamma-alumina" interface studied by diffuse reflectance spectroscopy (DRS)

Diffuse reflectance spectroscopy was used for the first time to investigate the adsorption of the [Co(H2O)6]2+ ions on the interface developed between the surface of the gamma-alumina particles and the electrolytic aqueous solutions used for the preparation of cobalt-supported gamma-alumina catalysts by equilibrium deposition filtration. The formation of inner-sphere Co(II) surface complexes in which Co(II) is in octahedral symmetry was confirmed. A deconvolution peak centered at approximately 585 nm was attributed to the exchange of one aqua ligand with one AlxOHy (x = 1, 2, or 3; y = 0 or 1) negatively charged surface group resulting in the formation of mononuclear monosubstituted inner-sphere Co(II) complexes at a Co(II) surface concentration equal to 0.02 micromol of Co(II)/m2. It was inferred that as the surface Co(II) concentration increases the formation of disubstituted and/or trisubstituted surface complexes is favored with respect to the formation of monosubstituted Co(II) surface complexes. A deconvolution peak centered at approximately 640 nm was attributed to the exchange of one or more aqua ligands with bridging hydroxo ligands (Co-O-H). The relative magnitude of this peak increases with the Co(II) surface concentration, reflecting the increasing formation of binuclear, oligonuclear, and multinuclear Co(II) surface complexes and then the formation of the Co(II) surface precipitate.     

Electrokinetic Separation Methods

Electrophoresis is the transport of dissolved molecules or suspended particles in a homogeneous polar liquid (such as water) under the influence of an electric field. Most molecules or particles acquire a surface electric charge when dissolved or suspended in buffered water (or other polar liquids), owing to ionization or adsorption of ions present in the water. The sign of the surface charge of molecules or particles determines whether they will migrate towards the positive or the negative electrode of the applied electric field, and the velocity of migration depends on the surface potential of the molecules or particles, as well as on the potential of the electric field.     

Adsorption of lysozyme to phospholipid and meibomian lipid monolayer films

It is believed that a lipid layer forms the outer layer of the pre-ocular tear film and this layer helps maintain tear film stability by lowering its surface tension. Proteins of the aqueous layer of the tear film (beneath the lipid layer) may also contribute to reducing surface tension by adsorbing to, or penetrating the lipid layer. The purpose of this study was to compare the penetration of lysozyme, a tear protein, into films of meibomian lipids and phospholipids held at different surface pressures to determine if lysozyme were part of the surface layer of the tear film. Films of meibomian lipids or phospholipids were spread onto the surface of a buffered aqueous subphase. Films were compressed to particular pressures and lysozyme was injected into the subphase. Changes in surface pressure were monitored to determine adsorption or penetration of lysozyme into the surface film. Lysozyme penetrated a meibomian lipid film at all pressures tested (max = 20 mN/m). It also penetrated phosphatidylglycerol, phosphatidylserine or phosphatidylethanolamine lipid films up to a pressure of 20 mN/m. It was not able to penetrate a phosphatidylcholine film at pressures ≥10 mN/m irrespective of the temperature being at 20 or 37 °C. However, it was able to penetrate it at very low pressures (<10 mN/m). Epifluorescence microscopy showed that the protein either adsorbs to or penetrates the lipid layer and the pattern of mixing depended upon the lipid at the surface. These results indicate that lysozyme is present at the surface of the tear film where it contributes to decreasing the surface tension by adsorbing and penetrating the meibomian lipids. Thus it helps to stabilize the tear film.     

Monochromatized x‐ray photoelectron spectroscopy of the AM60 magnesium alloy surface after treatments in fluoride‐based Ti and Zr solutions

The behaviour of the 6% aluminium–magnesium alloy (AM60) surface in zirconium or titanium fluoride aqueous acid solutions was studied. X‐ray photoelectron spectroscopy was used to investigate modifications in the surface chemistry with respect to the composition of the surface treatment solution. The surface film is composed of magnesium hydroxide and hydroxyfluoride, zirconium oxide, oxyhydroxide or oxyfluoride, titanium oxide and structural and adsorbed water. Optimal parameters leading to the formation of a zirconium‐ or titanium‐rich film were determined. A mechanism is proposed for the formation of zirconium‐ or titanium‐based films. Copyright © 2005 John Wiley & Sons, Ltd.     

Structural and shear characteristics of adsorbed sodium caseinate and monoglyceride mixed monolayers at the air-water interface

The structural and shear characteristics of mixed monolayers formed by an adsorbed Na-caseinate film and a spread monoglyceride (monopalmitin or monoolein) on the previously adsorbed protein film have been analyzed. Measurements of the surface pressure (pi)-area (A) isotherm and surface shear viscosity (eta(s)) were obtained at 20 degrees C and at pH 7 in a modified Wilhelmy-type film balance. The structural and shear characteristics of the mixed films depend on the surface pressure and on the composition of the mixed film. At surface pressures lower than the equilibrium surface pressure of Na-caseinate (at pipi(e)(CS) have important repercussions on the shear characteristics of the mixed films.     

聚醚硅油表面迁移行为对聚苯乙烯表面极性的影响

利用变角衰减全反射傅立叶变换红外光谱(ATR-FTIR)法和接触角,分析了聚醚硅油在聚苯乙烯共混物薄膜表面的选择性富集行为及对其表面结构和表面极性的影响,认为接触介质的表面性质是影响共混物中各组分产生选择性迁移扩散的重要影响因素.强极性介质的诱导作用可以在共混物表面层中产生剧烈变化的浓度梯度,而弱极性介质所产生的表面浓度梯度比较缓和.     

Contact-angle hysteresis on super-hydrophobic surfaces

The relationship between perturbations to contact angles on a rough or textured surface and the super-hydrophobic enhancement of the equilibrium contact angle is discussed theoretically. Two models are considered. In the first (Wenzel) case, the super-hydrophobic surface has a very high contact angle and the droplet completely contacts the surface upon which it rests. In the second (Cassie-Baxter) case, the super-hydrophobic surface has a very high contact angle, but the droplet bridges across surface protrusions. The theoretical treatment emphasizes the concept of contact-angle amplification or attenuation and distinguishes between the increases in contact angles due to roughening or texturing surfaces and perturbations to the resulting contact angles. The theory is applied to predicting contact-angle hysteresis on rough surfaces from the hysteresis observable on smooth surfaces and is therefore relevant to predicting roll-off angles for droplets on tilted surfaces. The theory quantitatively predicts a "sticky" surface for Wenzel-type surfaces and a "slippy" surface for Cassie-Baxter-type surfaces.     

Surface charge density/surface potential relationship for a spherical colloidal particle in a salt-free medium

On the basis of a theory of Imai and Oosawa (Busseiron Kenkyu52, 42 (1952); 59, 99 (1953)), approximate analytic expressions for the surface charge density/surface potential relationship for a spherical colloidal particle in a salt-free (aqueous or nonaqueous) medium containing only counterions are derived. There is a certain critical value of the surface charge density (or the total surface charge) separating two distinct cases: low surface charge density case and high surface charge density case. In the latter case counterion condensation occurs in the vicinity of the particle surface. The results are in excellent agreement with numerical calculations for the case of dilute suspensions.     

水在石墨(0001)面簇模型桥位上吸附的量子化学研究

用从头计算方法对水在石墨(0001)面桥位上的吸附进行了研究.用C₆H₈原子簇模拟石墨表面,在6-31G^*水平上计算了水在不同方向和位置上的吸附能量.研究表明:水在石墨面上的吸附很弱,属于物理吸附;在中性或带负电荷的石墨表面,当水分子中的氢原子靠近石墨面时,体系存在能量最小值,而在带正电荷的表面,当氧原子靠近石墨面时存在稳定的吸附点;不论表面带正电荷还是带负电荷,均对水分子的吸附起增强作用.     

Au(111)表面上乙醇选择性氧化反应机理的密度泛函理论研究(英文)

采用密度泛函理论研究了吸附有O原子的Au(111)表面上乙醇选择性氧化的反应机理.反应结果表明,除O原子和中间产物二齿醋酸根(CH3CHOO)外,其他中间产物在Au(111)表面扩散能垒均较低,不会对反应速控步骤的确定造成影响.乙醇羟基氧化脱氢为反应的第一步骤,当氧化剂为吸附态的O原子或者为OH基时,反应活化能分别为0.20和0.17eV.氧化产物乙氧基(CH3CH2O)进一步氧化脱氢生成乙醛则需要表面吸附的O原子或另一表面吸附的OH基的参与,所需活化能为0.29或0.27eV.同时,乙醛易与表面吸附的乙氧基反应生成乙氧基半缩醛(CH3CHOOC2H5),其可进一步与O原子作用,脱氢形成乙酸乙酯.此外,在乙醛深度氧化成酸的过程中需要克服较高的反应能垒,因而在表面反应温度较低时无法进行,这与实验结果相符.     

Reactivity at the mineral-water interface: dissolution and inhibition

The reactivity of the mineral-water interface is often interpreted with the help of models used in electrochemistry, in solution coordination chemistry and in crystallography. Progress in understanding mechanisms of growth and dissolution of crystals and the inhibition of these processes depends on a better integration of these models. It is shown that dissolution can be explained in terms of a ligand exchange process; simplified rate laws for proton- and ligand-protonated dissolution rates, being related to surface bound protons and ligands, respectively, can be derived. Further refinement in interpreting surface reactivity comes from an appreciation of the molecular structures at the mineral water interface; here significant advances have been made by X-ray absorption spectroscopy, especially EXAFS (Extended X-ray Absorption Fine Structure spectroscopy), which permit distinction to be made between outer-sphere and inner-sphere surface complexes, and in many cases to determine the structure of the surface species at different crystallographic planes (e.g., bi-nuclear or mono-nuclear linkage of ligands on metal ions to surface metal centers). Such information coupled with solution-chemical studies on the extent of adsorption can provide new insight into the mechanisms of dissolution reactions and their inhibition and surface poisoning. A few experimental results are given to exemplify the factors that enhance and inhibit the non-reductive (EDTA) and reductive dissolution (by H₂S) of Fe(III)(hydr)oxides. Binuclear surface complexes by multivalent cations and by oxoanions, such as phosphate, arsenate and borate, are believed to be efficient inhibitors for oxide dissolution because they form bi- or multinuclear innersphere surface complexes that can bridge two or more metal centers in the surface lattice; the simultaneous removal of such bi- or multinuclear surface complexes from the surface is energetically unfavorable. Proton and ligand promoted dissolution reactions and their inhibition by oxoanions and bi- or multinuclear surface complexes are not only relevant in geochemistry (weathering, soil-formation, transfer of elements and pollutants) but also in metallic corrosion, formation and breakdown of passive oxide films.