Formation Of Surface Phase Research Articles

Electrochemical Activation of Glassy Carbon: Spectroscopic Ellipsometry of Surface Phase Formation

The surface layer formed on glassy carbon (GC) electrodes during electrochemical activation was studied for the first time by in situ spectroscopic ellipsometry in 1M for two different activation procedures. The activation procedure consisted either of a certain number of potential cycles between −0.5 and 1.86 V (SCE) with a scan rate of 150 mV/s or of applying an anodic potential of 1.86 V (SCE) for a certain time. The spectroscopic ellipsometry results can be well described by assuming a porous hydrated surface film, consisting of a mixture of GC and electrolyte on top of the GC substrate. The film thickness increases monotonically with the duration of the activation process. Cycling of the electrode yielded films with constant void fraction (electrolyte) of about 80% while constant potential oxidation gave films with increasing void fraction between 0% and 70%. Upon removal from the electrolyte the surface film collapses which is evidenced by a significant decrease in thickness and void fraction. On the basis of our ellipsometry results a diffusion coefficient of can be determined for the intrinsic redox processes in the activated surface layer. A charge density of 250 Ah/dm3and 210 Ah/kg can be calculated for the active layer on activated glassy carbon electrodes, which is of importance for potential battery or capacitor applications.

Elemental Depth Profiling of Nuclear Waste Glasses after Two‐Years Burial in a Salt Geology

Detailed surface compositional analysis has been conducted on 12 simulated nuclear waste forms leached for two years in the brine environment of the Waste Isolation Pilot Plant (WIPP). Secondary ion mass spectrometry (SIMS) of both cations and anions has identified characteristic leaching and precipitation zones within the leached layers on all specimens. The thickness of the leached layers is found to range from approximately 0.3 to 2.7 μm and the total element loss from 80 to 625 mmol/m2, indicating that durability in brine differs by less than an order of magnitude between the different glasses. The in‐depth profiles of anionic elements indicate the significant roles of carbon, fluorine, and chlorine in the formation of surface phases. Also recorded and discussed are elemental SIMS images of sputter crater tapers, providing additional documentation of leaching subzones.

Si solid phase epitaxy on Si-Sb and Si-B surface phases

Results of LEED-AES study of the formation of buried Si-Sb and Si-B surface phases in Si(100) and Si(111) are presented, including preparation and characterization of surface phases, estimation of the crystalline quality of epitaxial Si films grown on surface phases, and examination of smearing of sharp concentration profiles in the growth direction subsequent to silicon epitaxial overgrowth.

Surface composition of stainless steel during active dissolution and passivation.

Anodic segregation of selected alloy constituents is found by ESCA analysis to occur as a result of active dissolution and during the passivation of stainless steel in deaerated acid. The resulting surface composition of the steel is radically different from the bulk and conditions seem to favour the formation of surface phases of intermetallic compounds. The resulting surface phases appear to control the kinetics of active dissolution and the composition of the passive film. In this respect we propose that in the more complex stainless steels passivation may be due to the formation of a duplex kinetic barrier consisting of an outer oxide based passive film and an inner intermetallic surface phase having a higher lattice binding energy than the bulk alloy. This paper will focus on the influence of anodically segregated Ni, Mo and N in a range of austenitic stainless steels exposed to deaerated HCl solutions.

Surface segregation of defects in oxide ceramic materials

It is documented that composition and crystal ordering of oxide surfaces and grain boundaries differ from the bulk as a result of segregation of lattice defects. Theoretical approaches on segregation in metals and limitations of their application to ionic solids are discussed. Several experimental approaches to evaluate segregation in ionic solids are considered. The approaches involve mainly the studies of surface (near-surface) versus bulk studies of several structural and composition-sensitive parameters as diffusion coefficient, Fermi energy level and chemical composition data. The oxygen-pressure exponent of defect-related parameters such as electrical conductivity, Seebeck coefficient, work function, weight change is also a sensitive probe in studies of segregation. Finally when the grain size decreases below a certain critical value, which is comparable to dimensions of the near-surface or grain boundary layer, then most of physical and chemical parameters are partially or entirely sensitive to properties of the layer. Examples are given to illustrate segregation in nonstoichiometric oxides and their solid solutions. The results are discussed in term of different bulk versus near-surface miscibility ranges and resulting impact on interpretation of phase diagrams for ionic solids. It is shown that segregation leads to formation of surface or grain boundary bidimensional phases which exhibit extraordinary properties. Segregation and thus formed strong electric fields lead to the formation of the near-surface diffusive resistance for transport of charged defects across interfaces. The resistance may have a significant effect on the rate of heterogeneous processes even at high temperatures. Practical aspects of segregation on chemical processes and materials properties are discussed.

Structure and properties of ultrathin iron films on [formula omitted]: The formation of metastable surface phases of γ-Fe

The chemisorption and desorption of Fe at the (10 10) surface of Ru has been investigated by LEED, Auger spectroscopy, Δφ and thermal desorption measurements over a substrate temperature range of 300–900 K. The growth mode of the iron deposit was found to be strongly dependent on the temperature. At 300 K up to seven iron monolayers could be grown, these adopting the configuration of the fcc (111) plane of bulk γ-iron. The layers were metastable and heating of such films or deposition at elevated temperatures resulted in nucleation and growth of crystallites. Only a single Fe desorption peak ( E d ≈ 250 kJ mol−1) was observed over the whole coverage regime; this is assigned to the evaporation of iron atoms in contact with the ruthenium substrate, either pre-existing in the first monolayer or supplied from the Fe crystallites.

Formation of a Metastable Ordered Surface Phase Due to Competitive Diffusion and Adsorption Kinetics: Oxygen on Pd(100)

Formation of a Metastable Ordered Surface Phase Due to Competitive Diffusion and Adsorption Kinetics: Oxygen on Pd(100)

The formation of InSi surface phase

The study of thin indium films on Si(1 1 1) using LEED, AES and ELS techniques was carried out. The ELS data were interpreted by means of the bulk plasmons theory. The formation of InSi surface phase both at RT and as a result of annealing was shown.

On the electrosorption of oxygen species and adlayer growth on noble metal surfaces

The surface chemistry, i.e. adsorption and surface phase formation, on Rh, Ir, Pt and Au single-crystal electrodes in 0.5 M H 2SO 4 has been studied in the potential range between 0 and 4 V vs. a standard hydrogen electrode by X-ray photoelectron spectroscopy and cyclic voltammetry. Both methods gave complementary results which were interpreted as potential-dependent changes from physisorption of water to adsorption of hydroxyl groups, hydroxide film formation, and eventual growth of thick adlayers of oxyhydroxide (Rh, Ir and Au) or hydroxide (Pt) with increasing applied voltage.

Ion mixing effects on the electrochemical behavior of tin coated nickel

Nickel substrates were coated with a thin Sn layer and implanted with 100 keV xenon ions, leading to the formation of new surface phases, mainly Ni 3Sn. Electrochemical measurements were carried out on Ni samples submitted to each of the following surface modifications: Sn-deposition, Xe +-implantation, ion-mixing (combined treatment), and on untreated Ni. The test solution was 0.5 M NaCl. Both the potentiodynamic anodic and cathodic polarization curves show that the implantation on uncoated Ni improves the anodic passivation with respect to the Sn deposition on Ni; comparable results are obtained by surface alloying induced by ion-mixing. The cathodic behaviour is in agreement with the presence of both galvanic couples and active sites for H 2 evolution due to defect spreading. Only the alloyed surface shows a clearly different behaviour, i.e. a high hydrogen reduction overvoltage. The electrochemical measurements were repeated in the presence of sulphide ions.

Microscopic effects at GaAs/Ge(100) molecular-beam-epitaxy interfaces: Synchrotron-radiation photoemission study.

We have measured the evolution of abrupt GaAs/Ge(100) interfaces to study the relationship between the valence-band discontinuity (\ensuremath{\Delta}${E}_{v}$) and growth properties on an atomic scale. We used soft x-ray photoemission, low-energy electron diffraction, and Auger electron spectroscopy. The interface was obtained by evaporating Ge (at ${10}^{\mathrm{\ensuremath{-}}9}$ Torr and intentionally coevaporating at an overpressure of ${\mathrm{As}}_{4}$) in situ onto a molecular-beam-epitaxy- (MBE-) grown GaAs sample under epitaxial growth conditions. The MBE-grown GaAs(100) provided different surface reconstructions with controllable starting anion-to-cation ratio. We present a new core-intensity-analysis method for determining thin-film growth independent of the adlayer thickness calibration. For all interfaces, a monolayer of segregated As was observed at the free Ge surface. This suggests that the As-stabilized Ge surface phase formation plays an important role in determining the interface growth. The value we obtain for \ensuremath{\Delta}${E}_{v}$ is (0.47\ifmmode\pm\else\textpm\fi{}0.05) eV, independent of both the initial clean GaAs(100) surface properties and the evolution of the Fermi level. We propose that the driving force for MBE interface formation always yields a unique, low-energy equilibrium structure at the materials transition region, although its extent may vary by as much as a couple of atomic layers. However, the observed constancy of \ensuremath{\Delta}${E}_{v}$ suggests that the contribution of any perturbation caused by variations in the local microscopic properties of the interface (e.g., dipoles) is very small compared to the intrinsic (perhaps bulk) mechanism that determines \ensuremath{\Delta}${E}_{v}$.

Multilayer effects in liquid adsorption chromatography with mixed mobile phases

AbstractThis paper describes the theory of liquid adsorption chromatography (LSC) with mixed mobile phase, involving formation of multilayer surface phases. An equation describing the dependence of the capacity ratio upon the mobile phase composition is derived and theoretical curves illustrating the same dependence are calculated accordingly by assuming different thicknesses of the surface phase.

Application of the phase rule to heterogeneous chemisorption systems

The possibility to apply a generalized phase rule to the formation of bulk and surface phases under equilibrium chemisorption and catalysis is discussed.

Ancient Marine Phosphorites

Marine phosphorites have been studied for more than a hundred years, both because of their commercial value as fertilizer and because of their importance to sedimentary petrology. Phosphorus is, of course, one of the basic plant nutrients and is intimately involved in biospheric processes ; however, its role in biogenic and chemogenic sedimentation is difficult to study and is not completely understood. Marine phosphorites on the seafloor today occur on continental shelves, rises, and seamounts where they are difficult to observe and collect. In the laboratory, the mineral structure and crystal chemistry of sedimentary apatite has been difficult to analyze because of its complexity and small crystal size, and apatite has not yet been synthesized at low temperatures in solutions that approx­ imate seawater in composition. Physical-chemical equilibrium studies have been difficult because of the slow rates of reaction, the complex composition of apatite, and the formation of complex surface phases and complex phosphorous ions in solution, as well as the complex nature of seawater itself. As a result, progress has been slow in understanding the origin of sedimentary phosphorites, including the source and the processes of concentration of the phosphorus and the processes and the environment of deposition of the phosphorite. Hypotheses and arguments have pro­ liferated; however, in recent years consensus has been reached on many points. Arguments still exist on a number of other points, stimulating continued research in the field. Sedimentation processes commonly can be studied directly in the modern environment, and the results can be applied by analogy to inter­ pret ancient deposits according to Lyell's principle. In the case of phos­ phorites, marine geology studies, particularly in the last decade, have produced information that has been extremely interesting and somewhat

Co-adsorbed phases of hydrogen and oxygen on W(001)

LEED studies of the co-adsorption of hydrogen and oxygen on tungsten (001) have revealed the formation of distinct surface phases. At room temperature and a total coverage of about one-half monolayer the adsorption is competitive: oxygen and hydrogen produce separate domains on the surface. When the crystal is heated, a homogeneous, mixed phase is produced. The mixing is reversible and no hysteresis is observed upon cooling. The transition occurs at a temperature substantially below the disordering temperature for the pure oxygen structure; it is ascribed to an effective repulsion between adsorbed H and O. Studies of the adsorption rate show that in this coverage range, the hydrogen sticking probability is independent of any preadsorbed oxygen.

Novel surface phase containing cholesteryl esters. 2. Nonequivalence of cholesteryl arachidonate and those with 18-carbon, cis-unsaturated acyl groups.

Surface pressure--area isotherms for binary mixtures of cholesteryl octanoate, elaidate, stearate, oleate, linoleate, linolenate, and arachiodonate in mixtures with dioleoyllecithin, triolein, oleic acid, and oleoyl alcohol were measured at 24 degrees C. Analysis of the pressure and area characteristics as a function of composition showed that double-layer surface phase formation is primarily dependent on the structure of the acyl moiety of the cholesteryl ester. Cholesteryl esters with saturated or trans-unsaturated acyl chains apparently do not form double-layer surface phases. The esters of oleate, linoleate, and linolenate formed double-layer as well as monolayer phases and their properties in these phases were similar. In contrast to other cis-unsaturated esters, cholesteryl arachidonate formed a mixed monolayer phase with miscibility in all proportions and did not form a double-layer phase. Our results show that the polar lipid monolayer separating bulk cholesteryl ester from the aqueous milieu not only solubilizes finite amounts of cholesteryl esters but also can contribute to the organization of lipid adjacent to the monolayer. That such organization is observed with the predominant cholesteryl ester species of blood and aorta suggests a role for double-layer structure in regulating the transport and metabolism of cholesteryl esters in lipoproteins, arterial lipid deposits, and adrenal cortex. The absence of double-layer formation and high monolayer solubility of cholesteryl arachidonate suggest that it should be more abundant than other cholesteryl esters in bilayers and in monolayers surrounding bulk lipid phases.

The effect of absorbed oxygen on the kinetics of chemical reactions on metal surfaces

The effects of adsorbed impurities on the kinetics of reactions on metal surfaces are discussed, in particular the reactions of water vapor and carbon dioxide with iron surfaces are analyzed. The critical oxygen potentials for the onset of ‘poisoning’ of the reactions appear to correspond with the onset of facetting of iron and the formation of surface phases on the iron. The maximum chemical reaction rate constants for the reactions of water vapor and of carbon dioxide on iron surfaces at a given temperature are shown to correspond closely to the chemical reaction rate constants determined from the reduction of iron oxides in their respective systems. The phenomena observed on the reaction of gases with iron and iron oxide surfaces are reported in other metal systems thus the analysis used for the present reactions appears to be applicable to other systems involving metal/gas reactions.

Abstract: Study of semiconductor surfaces using an atom-probe field ion microscope (1) Hydrogen chemisorption on silicon

Semiconductors, particularly silicon, are some of the most important materials in technology today. Surface properties of Si have been intensively investigated by many techniques, such as low-energy electron diffraction (LEED)1,2 and ultraviolet photoemission spectroscopy (UPS).3,4 Most recently Sakurai and Hagstrum have studied surface electronic structures for hydrogen chemisorption on various silicon surfaces using ion neutralization spectroscopy (INS) and UPS.5 Their study suggested the formation of new hydride surface phases, such as SiH5b and SiH3,5c in addition to a well known monohydride phase SiH.5a However this conclusion was drawn without observation of those surface molecules. Here we present a new approach to the study of hydrogen chemisorption on Si surfaces. We applied atom-probe field ion microscopy to observe directly these surface compounds. We have also succeeded in achieving a new atomic resolution of silicon in field ion microscopy.

Reduction kinetics of a chalcopyrite electrode surface

The first cathodic linear sweep voltammogram with a freshly-polished chalcopyrite (CuFeS 2 ) electrode shows a small peak (prewave) preceding the main voltammetric wave for reduction to chalcocite (Cu 2 S). The charge under the peak is of the order of 1 mC cm −2 and is independent of sweep rate. Current-time curves at potentials in the prewave region have maxima, indicative of surface phase formation involving expansion and overlap of growing nuclei. The results are interpreted in terms of nucleation and two-dimensional phase growth of a thin product layer prior to the bulk reduction of chalcopyrite to chalcocite. The possible identity of the product phase is discussed.

Reduction kinetics of a chalcopyrite electrode surface

The first cathodic linear sweep voltammogram with a freshly-polished chalcopyrite (CuFeS 2) electrode shows a small peak (prewave) preceding the main voltammetric wave for reduction to chalcocite (Cu 2S). The charge under the peak is of the order of 1 mC cm −2 and is independent of sweep rate. Current-time curves at potentials in the prewave region have maxima, indicative of surface phase formation involving expansion and overlap of growing nuclei. The results are interpreted in terms of nucleation and two-dimensional phase growth of a thin product layer prior to the bulk reduction of chalcopyrite to chalcocite. The possible identity of the product phase is discussed.