Outermost Surface Layer Research Articles

Resonant dissociation of polyatomic surface molecules by hyperthermal ions

A theoretical approach is suggested for the fragmentation of polyatomic surface molecules by scattering of hyperthermal atomic and molecular ions. It is assumed that the surface molecules contain chains of identical diatomic valence groups as substructures. These chains can accumulate collective vibrational excitations (excimols), leading to subsequent nonstatistical molecular fragmentation. Low-energy incident ions at any incident angle are assumed to be reflected from the surface at atomic distances by the Coulomb potential, which acts for only a short period of time (10−13–10−12s). During this reflection period, the component of ion velocity normal to the surface is negligible, so ions move across the surface in a process that can be considered as a grazing collision. Analytical expressions are obtained for the fragmentation probabilities of surface molecules in grazing collisions with atomic and molecular ions. The probability of these processes strongly depends on the molecular structure and the incident ion velocity. The interaction between hyperthermal ions and molecules forming the outermost surface layer yields fragment molecules as a result of the mutual excitation processes. The experimental data on fragments generated from a fluorinated surface by action of incident ions are analyzed within the framework of the suggested model. Copyright © 2001 John Wiley & Sons, Ltd.

Resonant dissociation of polyatomic surface molecules by hyperthermal ions

AbstractA theoretical approach is suggested for the fragmentation of polyatomic surface molecules by scattering of hyperthermal atomic and molecular ions. It is assumed that the surface molecules contain chains of identical diatomic valence groups as substructures. These chains can accumulate collective vibrational excitations (excimols), leading to subsequent nonstatistical molecular fragmentation. Low‐energy incident ions at any incident angle are assumed to be reflected from the surface at atomic distances by the Coulomb potential, which acts for only a short period of time (10−13–10−12s). During this reflection period, the component of ion velocity normal to the surface is negligible, so ions move across the surface in a process that can be considered as a grazing collision. Analytical expressions are obtained for the fragmentation probabilities of surface molecules in grazing collisions with atomic and molecular ions. The probability of these processes strongly depends on the molecular structure and the incident ion velocity. The interaction between hyperthermal ions and molecules forming the outermost surface layer yields fragment molecules as a result of the mutual excitation processes. The experimental data on fragments generated from a fluorinated surface by action of incident ions are analyzed within the framework of the suggested model. Copyright © 2001 John Wiley & Sons, Ltd.

The effect of Ni content on the electrochemical and surface characteristics of Mg 90− xTi 10Ni x ( x=50, 55, 60) ternary hydrogen storage electrode alloys

Mg-based ternary Mg 90− x Ti 10Ni x ( x=50, 55, 60) electrode alloys of different Ni content, were prepared by mechanical alloying (MA) in this work. The main phase of these alloys is amorphous according to the XRD analysis. The electrochemical test indicates that the cycling stability of the electrodes made of these alloys improves with increasing Ni content, while the initial discharge capacity decreases dramatically. XPS analysis reveals that the outmost surface layer on the Mg–Ti–Ni alloys is a Mg(OH) 2 passive film, in which the content of Mg increases, and the degree of oxidation of Mg becomes higher as the cycling goes on. Below the Mg(OH) 2 film is a composite layer of several oxides including NiO, TiO 2 and Mg(OH) 2. This layer is insoluble and compact, and helps to inhibit further corrosion of the fresh alloy surface underneath. In this work, it was also found that with increasing Ni content, the high-rate dischargeability (HRD) and the exchange current density of the ternary Mg–Ti–Ni alloys are significantly increased. Metallic Ni particles generated by reduction from NiO during charging are regarded as the cause for the improved electrochemical activation.

A histochemical study of the secretory gland cells of Cercaria shikokuensis and their role during development from cercaria to metacercaria

The roles of secretory glands during the developmental process from an immature cercaria to a metacercaria in Cercaria shikokuensis were studied. Four types of secretory cells were identified in this species. On maturation of the cercaria in redia, the products of ventral gland cells and mucoid gland cells formed a thick surface coat on the mature cercaria, and the products of cephalic gland cells also formed a thin cover on the surface coat. In the process leading to the formation of a metacercaria, the surface coat constituted the outer layer of the cyst, mucoid gland cells secreted mucous substances inside the wall, and then cystogenous gland cells discharged their products to the inner wall. The cyst wall was composed of four layers, and it was thought that the outermost surface layer helped the cyst wall to adhere to the matrix and the intermediate layers helped to put together outer and inner walls.

An atomic structural model of ( [formula omitted])- R30° reconstruction proposed for 3C–SiC(1 1 1) crystallized islands on Si(1 1 1) by C 60 precursor

The surface structures of 3C–SiC(1 1 1) crystallized islands formed by thermal chemical reaction between silicon substrate and adsorbed C 60 molecules at high temperatures are studied using ultrahigh vacuum scanning tunneling microscopy (UHV-STM). The STM images show a strong bias voltage dependence, transforming between (2×2), (2 3 ×2 3 ) - R30° and Y-type patterns under different bias voltages. A surface atomic structural model containing quarter coverage of vacancies and three different sites of surface silicon atoms is proposed to interpret the experimental observations. The dangling bond electronic states contributed by these different site Si atoms on the outermost surface layer and by the carbon atoms surrounding the vacancies are suggested to be responsible for the different tunneling conditions under different bias voltages in STM images.

Possible effects of site isolation in antimony oxide-modified vanadia/titania catalysts for selective oxidation of o-xylene

Titania-supported vanadia catalysts were modified by addition of antimony oxide for application in o-xylene selective oxidation to phthalic anhydride. It was shown that active and selective catalysts can be prepared by ball-milling mixtures of powders of TiO2, V2O5and Sb2O3followed by calcination. X-ray photoelectron spectroscopy proves the formation of highly dispersed overlayers of vanadium oxide and antimony oxide, in which V5+is partially reduced to lower oxidation states and Sb3+is partially oxidized to Sb5+. Antimony oxide segregated into the outermost surface layers. It is therefore inferred that the presence of the antimony oxide modifier spatially separates V–O species and leads to “site isolation” which may be responsible for the positive effect of the modifier for the catalyst's selectivity.

Application of ultraviolet photoelectron spectroscopy (UPS) in the surface characterisation of polycrystalline oxide catalysts. I. Methodics and analytical potential

Polycrystalline oxide surfaces have been investigated by a combination of photoemission and ion-scattering techniques (UPS, XPS, ISS) to assess the analytical potential of UPS for the surface characterisation of real polycrystalline oxide catalysts. UP spectra can be obtained from a number of binary and ternary oxides when the measurement is performed at appropriate temperatures in order to ensure sample conductivity sufficient to avoid surface charging. Where applicable, UPS provides enhanced surface sensitivity (as compared to XPS) and chemical sensitivity permitting the identification of surface structures and adsorbates. The benefit of enhanced surface sensitivity has been demonstrated by the detection of V4+ species in the outmost surface layer(s) of V2O5. The use of the chemical sensitivity for the identification of chemical compounds on a fingerprint basis has been shown by a study of bismuth molybdate surfaces. The chemical sensitivity with respect to adsorbates has been demonstrated by the detection of N-containing species formed in the interaction of V2O5 with NH3.

Evolução estrutural do filme cerâmico de forsterita obtido sobre o aço silício de grão orientado

Neste trabalho é descrita a formação, sobre o aço silício de grão orientado, de um filme cerâmico obtido a partir de MgO contendo os aditivos TiO2 e SrSO4. O material foi tratado termicamente a 900 0C, 1000 0C e 1200 0C e por 15 horas a 1200 0C. O recobrimento foi analisado por microscopia eletrônica de varredura (MEV), microssonda eletrônica (EDS), difração de raios X e espectroscopia de centelhamento (GDS). A evolução morfológica observada se passa com a formação de uma camada contínua de óxidos na superfície mais externa do material. Com a elevação da temperatura a camada mais externa torna-se mais espessa e aumenta o teor de Mg nas partículas presentes na sub-camada. É possível verificar a presença de forsterita cristalina na superfície das amostras tratadas a temperaturas superiores a 1000 0C. A profundidade de penetração do Mg no material aumenta com a elevação da temperatura, passando de cerca de 75 nm a 900 0C para aproximadamente 1,9 mim a 1200 0C.

Surface reactions of a plasma-sprayed CaO–P 2O 5–SiO 2-based glass with albumin, fibroblasts and granulocytes studied by XPS, fluorescence and chemiluminescence

X-ray photoelectron spectroscopy (XPS) was used to define the chemical composition of the outermost surface layer and the surface modification of a plasma-coated phospho-silicate glass (identified as BVA) when immersed in K-phosphate buffer or in phosphate buffered human albumin solution. Its behavior was compared with that of a soda-lime-based glass (identified as BVH) treated in the same way. The surface % composition of plasma-sprayed glass was consistent with bulk composition. After incubation with buffer, a Ca–P-rich layer developed only on the surface of BVA glass. Human serum albumin was bound reversibly to both glasses maintaining its native state. However, the protein completely covered the BVA glass surface within 24 h, with the formation of a mixed albumin–Ca–P layer, while 4 days incubation was necessary for complete coverage of BVH glass surface. Murine fibroblasts seeded on plasma-coated BVA glass showed a proliferation pattern similar to that of control cells grown on Petri dish, while cells seeded on BVH had more restricted growth. A limited response was induced in polymorphonuclear granulocytes by both bulk glasses powder. In conclusion, the glass identified as BVA has the suitable characteristics of its surface layers to be considered biologically active from both a chemical and a cellular point of view.

Application of Ultraviolet Photoelectron Spectroscopy in the Surface Characterization of Polycrystalline Oxide Catalysts. 2. Depth Variation of the Reduction Degree in the Surface Region of Partially Reduced V2O5

The surface of partially reduced V2O5 has been studied by photoemission techniques of different sampling depths (ultraviolet photoelectron spectroscopy (UPS), V (2p) X-ray photoelectron spectroscopy (XPS), and valence-band XPS excited by Mg Kα) in order to investigate the distribution of V4+ species in the near-surface region. Reduction was performed by evacuation in ultrahigh vacuum or treatment with flowing hydrogen (10% H2 in Ar) at temperatures between 373 and 923 K. Average reduction degrees were derived from V4+ (3d) and O (2p) signals in the valence-band region and from the V (2p3/2) signal (V4+/V5+ ratio). Comparison of average reduction degrees obtained from different sampling depths showed that pronounced depth profiles of the V4+ concentration develop during reduction. At low reduction temperatures, the reduction degree decreases monotonically with the distance from the surface. At higher temperatures, when oxygen mobility in the solid competes with oxygen removal from the surface, the depth profile of the reduction degree changes significantly. During reduction in flowing hydrogen, the maximum of the reduction degree shifts from the outermost surface layer into subsurface layers. A similar effect is likely to occur during reduction by thermal evacuation. The strong inhomogeneities in the near-surface region may lead to significant errors when the reduction state of the outermost surface layer is described on the basis of XPS alone under the assumption of a homogeneous sampling region. This indicates a need for further improvement of the methodology for surface analysis with reduced bulk oxides.

Macromolecular transport across the superficial layer of articular cartilage

Objective To study the role of the superficial layer of articular cartilage in the transport of macromolecular solutes.Design The articular cartilage of intact bovine carpal bones was incubated with125I-labeled bovine serum albumin, human IgG, or horse ferritin for 4 hours. Quadruplicate samples were first incubated with polymorphonuclear neutrophil elastase for 30 minutes to remove the outermost layer covering the articular surface. The rates of exchange of each macromolecule from excised tissue explants in the absence of a concentration gradient were measured at six different time points. The results were expressed as the fraction of radioactive protein exiting the cartilage per mm2of tissue, or as picomoles of labeled solute per mm2.Results Exchange rates correlated well with molecular mass, and no apparent differences were detected between intact and elastase-treated tissues. However, when the results were expressed in terms of the total number of molecules within the tissue, it was apparent that IgG molecules accumulated in the intact cartilage in larger than expected numbers. This finding was not observed in experiments using elastase-treated tissue.Conclusion These observations suggest that the outermost surface layer does not constitute a barrier to the transport of macromolecules into the deeper zones of the tissue. The higher IgG accumulation observed in intact cartilage suggests that the acidic outer layer of cartilage exhibited attractive interactions, probably ionic in nature, with the cationic fraction of IgG. These observations may relate to our previous work demonstrating that the sequestered immune complexes in the superficial zone of articular cartilage in rheumatoid arthritis, and in the antigen-induced arthritis model, are formed because pre-existing antibody normally present in cartilage irreversibly traps antigen within the tissue.

Surface modifications of plasma-sprayed bioglasses upon interaction with phosphate buffer and albumin solutions: a photoelectron spectroscopy study

X-ray photoelectron spectroscopy (XPS) was used to define the chemical composition of the outermost surface layer and the surface modification of a plasma-coated phospho-silicate glass (identified as BVA) when immersed in potassium phosphate buffer or in phosphate-buffered human albumin solution. Its behaviour was compared with that of a soda-lime-based glass (identified as BVH) treated in the same way. The surface percentage composition of plasma-sprayed glass was consistent with the bulk composition. After incubation with buffer, a Ca–P-rich layer developed only on the surface of BVA glass. Human serum albumin was bound reversibly to both glasses; however, the protein completely covered the BVA glass surface within 24 h, with the formation of a mixed albumin–Ca–P layer, whereas 4 days of incubation were necessary for complete coverage of the BVH glass surface. The thickness of the organic/inorganic overlayer has also been estimated. Copyright © 2000 John Wiley & Sons, Ltd.

Hydrophobicity loss and recovery of silicone HV insulation

Most of the silicone materials used for Hv outdoor insulation are high-consistency, heat cured polydimethylsiloxane (PDMS) elastomers. The unique properties of PDMS that make it suitable for HV applications are reviewed. The surface of these elastomers can be rendered hydrophilic by exposure to discharges. A time and temperature dependent hydrophobic recovery ensues when exposure ceases. A variety of surface characterization investigations have established that corona exposure forms a brittle, wettable, silica-like layer on the surface of most PDMS elastomers. This is consistent with similar effects from oxygen and inert gas plasma treatment. There is still considerable debate as to the relative importance of the two major mechanisms postulated to account for the hydrophobic recovery after corona discharge. The diffusion mechanism invokes migration of low molecular weight species from the interior to the surface, while the reorientation or overturn mechanism envisages a surface reorganization with polar entities such as silanol groups resulting from surface oxidation rotating away and being replaced by methyl groups in the outermost surface layers. In our view, the highly crosslinked silicalike layer cannot reorient readily between hydrophilic and hydrophobic states at the surface, suggesting that diffusion of low molecular weight PDMS components is the more important mechanism of hydrophobic recovery. Recent data obtained on PDMS samples free from lowmolecular- weight diffusible species show that hydrophobic recovey may be due to in-situ depolymerization.

Controlled Etching of Au:SR Cluster Compounds

When Au:SR cluster compounds (R = (CH2)5CH3) with an Au metallic cluster core mass of ∼14 kDa (∼75 atoms, 1.1 nm equivalent diameter), are heated in neat dodecane−thiol solution under inert atmosphere, species with smaller Au core masses are formed, presumably by the removal of Au atoms from the outermost surface layer of the cluster's Au core. This process was monitored through laser desorption mass spectrometry, optical absorption spectroscopy, and X-ray diffraction, where all three methods indicate a substantial decrease (>50%, by mass or number of Au atoms) in the size of the cluster compound's inorganic core. The optical absorption spectra and laser desorption mass spectra of the compounds generated by etching are strikingly similar to compounds previously separated without an etching step. The dual function of the thiol as both stabilizing adsorbate and efficient etchant sets this cluster (or nanocrystal) system apart from other metallic and semiconductor systems.

First-principles theoretical studies of the structural, electronic and magnetic properties of Co{1010}-A

Abstract We present results of ab initio pseudopotential calculations for the {10 1 0}-A surface of the ferromagnetic 3d transition metal cobalt. Our calculated structural relaxation is found to be in good agreement with existing experimental results, albeit with slightly exaggerated changes relative to the bulk. The role of spin-polarisation is considered, and a 15.8% enhanced magnetic moment is observed on the outermost surface layer, in line with previous theory for more closely packed ferromagnetic surfaces. Despite the open nature of the surface, and the consequent distortion of their d-orbitals, the exposed second layer atoms display essentially bulk-like magnetic moments.

Reaction and surface characterization study of higher-alcohol synthesis catalysts XII: K- and Pd-promoted Zn/Cr/Mn spinel

The synthesis of higher alcohols from syngas is a potentially attractive route for the downstream production of methyl tertiary-butyl ether (MTBE), an oxygenated gasoline additive. One of the most effective types of higher alcohol synthesis (HAS) catalysts is a Zn/Cr spinel promoted with either Cs or K. Many studies have suggested that the Cr, required to form the high surface area spinel structure, is detrimental in HAS because it may enhance hydrocarbon formation. For this reason catalysts were examined in the present study in which Mn was substituted into the spinel thereby replacing some of the Cr. Two different K and Pd-promoted Zn/Cr/Mn spinel catalysts were tested for HAS using a 1:1 CO/H 2 syngas feedstream. These catalysts were promoted with 2.25 wt% K, 5.9 wt% Pd and 1.0 wt% K, 9.0 wt% Pd. The former performs better under all four reactor conditions examined (pressures of 1000 and 1500 psig and temperatures of 400°C and 440°C). At 440°C and 1500 psig the isobutanol production rate is 179 g/kg h, the total alcohol production rate is 304 g/kg h and no significant changes in the product stream composition occur over a five-day test period. The methanol-to-isobutanol mole ratio is 1.3, which is very close to the desired value of 1.0 for MTBE production. This is the most active HAS catalyst described in the literature. Surface characterization data, obtained using X-ray photoelectron spectroscopy and ion scattering spectroscopy, indicate that the Mn content in the near-surface region of the catalyst is small. The characterization data also demonstrate that the reductive pretreatments given to HAS catalysts prior to testing result in the elimination of C-containing contamination, adsorbed water and hydroxyl groups and either expose underlying K and Pd promotors or possibly induce the migration of the K and Pd to the outermost surface layers. The pretreated catalyst surface has a layered structure with an enriched K- and Pd-containing layer at the surface above the ZnO layer over the spinel support.

Wettability of poly(ethylene terephthalate) film treated with low‐temperature plasma and their surface analysis by ESCA

The surface of poly(ethylene terephthalate) (PET) film was modified by low-temperature plasma with O2, N2, He, Ar, H2, and CH4 gases, respectively. After being treated by low-temperature plasma, their surface wettability and chemical composition were investigated by means of electron spectroscopy for chemical analysis (ESCA) and contact angle measurement. The result shows that the surface wettability of PET can be improved by low-temperature plasma, and the effect of the modification is due mainly to the kind of the gases. Mainly because of the contribution of hydrogen bonding force γ[STACK]cS[ENDSTACK], the surface wettability of PET treated with O2, N2, He, and Ar plasma for a short time (3 min) increase sharply, and the surface wettability is also improved by H2 plasma treatment; but the CH4 plasma treatment does not improve the wettability of PET. ESCA shows that the effect of wettability of PET is tightly related to the presence of polar functional groups that reside in the outermost surface layer of PET. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1327–1333, 1999

Applicability of FTIR/PAS depth profiling for the study of coated papers

Depth profiling of the chemical composition of paper and paper coatings is of essential importance for the development of paper qualities tailored to the demands set by recycling and new printing technologies. In this work the chemical composition of biodegradable, starch-based paper coatings was investigated by Fourier transform photoacoustic spectroscopy (FTIR/PAS). The coatings were prepared by mixing solutions of cationised starch and anionic surfactant (sodium oleate) and were applied in thicknesses ranging from 20 to 60 μm. FTIR/PAS spectra were recorded using step-scan phase modulation, and the spectral information was processed using photoacoustic (PA) magnitude/phase spectra calculations and generalised two-dimensional (G2D) spectral correlation analysis. Variation of the phase modulation frequencies made it possible to differentiate the characteristic bands arising from the base paper and the coating layer. Closer study of the coating spectra showed, however, that the position of the carbonyl salt band of sodium oleate is influenced by the environment, i.e., the position of band is different in pure oleate, oleate mixed with starch and oleate-starch mixture in contact with the calcium carbonate-containing base paper. On the basis of this information, the specific depth profile of the coating layer was obtained by analysis of magnitude spectra, phase angles and a G2D correlation spectrum. Each of these approaches indicated that the long chain surfactant (sodium oleate) is enriched in the outermost surface layer of the coating. In the attempt to calculate the thickness of the coating layer from the phase data, the overlapping of the signals did not allow the traditional equation to be used as such and a new baseline correction method was introduced to eliminate the overlapping. The advantage of this method is that no measurements of pure components are required. However, the method can be applied only for well-resolved and linearly overlapped bands. FTIR/PAS is concluded to yield a completely new type of qualitative information, which can be utilised in the development of paper coatings.

Observation of outermost surface layer of 4-mercaptohydrocynnamic acid self-assembled film on Au(111) by Penning ionization electron spectroscopy

Penning ionization electron spectroscopy (PIES) and ultraviolet photoelectron spectroscopy (UPS) were used to characterize the self-assembled (SA) film of 4-mercaptohydrocynnamic acid deposited on Au(111) from aqueous solution. The analysis of the relative band intensity of the Penning spectra suggests that the molecules expose the thiophenyl group outside the film surface, and form a bilayer made up of a hydrogen-bonding network of carboxyl groups.

Wettability of poly(ethylene terephthalate) film treated with low-temperature plasma and their surface analysis by ESCA

The surface of poly(ethylene terephthalate) (PET) film was modified by low-temperature plasma with O2, N2, He, Ar, H2, and CH4 gases, respectively. After being treated by low-temperature plasma, their surface wettability and chemical composition were investigated by means of electron spectroscopy for chemical analysis (ESCA) and contact angle measurement. The result shows that the surface wettability of PET can be improved by low-temperature plasma, and the effect of the modification is due mainly to the kind of the gases. Mainly because of the contribution of hydrogen bonding force γ[STACK]cS[ENDSTACK], the surface wettability of PET treated with O2, N2, He, and Ar plasma for a short time (3 min) increase sharply, and the surface wettability is also improved by H2 plasma treatment; but the CH4 plasma treatment does not improve the wettability of PET. ESCA shows that the effect of wettability of PET is tightly related to the presence of polar functional groups that reside in the outermost surface layer of PET. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1327–1333, 1999