Initiation and inhibition of dealloying of single crystalline Cu3Au (111) surfaces.

Dealloying is widely utilized but is a dangerous corrosion process as well. Here we report an atomistic picture of the initial stages of electrochemical dealloying of the model system Cu(3)Au (111). We illuminate the structural and chemical changes during the early stages of dissolution up to the critical potential, using a unique combination of advanced surface-analytical tools. Scanning tunneling microscopy images indicate an interlayer exchange of topmost surface atoms during initial dealloying, while scanning Auger-electron microscopy data clearly reveal that the surface is fully covered by a continuous Au-rich layer at an early stage. Initiating below this first layer a transformation from stacking-reversed toward substrate-oriented Au surface structures is observed close to the critical potential. We further use the observed structural transitions as a reference process to evaluate the mechanistic changes induced by a thiol-based model-inhibition layer applied to suppress surface diffusion. The initial ultrathin Au layer is stabilized with the intermediate island morphology completely suppressed, along an anodic shift of the breakdown potential. Thiol-modification induces a peculiar surface microstructure in the form of microcracks exhibiting a nanoporous core. On the basis of the presented atomic-scale observations, an interlayer exchange mechanism next to pure surface diffusion becomes obvious which may be controlling the layer thickness and its later change in orientation.

Electrodeposition of Zn and Au-Zn alloys at low temperature in an ionic liquid.

In this communication, electrodeposition of Zn from 60-40 mol% ZnCl(2)-1-butyl-3-methylimidazolium chloride (BMIC) ionic liquid on Au substrates has been investigated. For the first time, initial stages of Zn electrocrystallization from BMIC has been studied by in situ X-ray diffraction (XRD) employing synchrotron radiation, which showed an initial epitaxial deposition of Zn and hexagonal Au(1.2)Zn(8.8) phases on Au(111) single crystal substrates. In the later stages of electrodeposition, phase analysis showed a formation of several Zn-Au intermetallics, namely AuZn, AuZn(3), and Au(1.2)Zn(8.8), along with the Zn phase.

Height Velocity in Apparently Healthy North Indian School Children.

OBJECTIVE: Linear growth is best estimated by serial anthropometric data or height velocity (HV). In the absence of recent data on growth velocity, we undertook to establish normative data in apparently healthy North Indian children. MATERIALS AND METHODS: Prospective longitudinal study in a representative sample of 7710 apparently healthy children, aged 3-17 years from different regions of Delhi. Height was measured at baseline and at 12 months while pubertal examination was performed at baseline in a subset of children. RESULTS: The data on HV and puberty were available in 5635 participants (73.08%; 2341 boys and 3294 girls) and 1553 participants (622 boys; and 931 girls), respectively. The mean peak height velocity (PHV) was 7.82 ± 2.60 cm in boys seen at 12-12.9 years and 6.63 ± 1.81 cm in girls at 10-10.9 years Although late maturing boys had a greater HV than early or normal maturers, it did not vary with the age of pubertal maturation in girls. HV correlated with parental height in prepubertal boys, girls, and pubertal boys (P < 0.01) while no correlation was seen in girls. CONCLUSIONS: The study presents normal height velocities in North Indian children. A secular trend was observed in achieving PHV in both boys and girls.

Competitive adsorption of glycine and water on the fluorapatite (100) surface.

The atomic structure of the aqueous glycine-fluorapatite (100) interface was investigated using grazing incidence X-ray diffraction. Experimental data analysis of crystal truncation rod intensities revealed detailed information on lateral as well as perpendicular ordering of the adsorbate molecules and the nature of atomic relaxations in the fluorapatite (FAp) (100) surface. Glycine and water molecules are arranged in two periodically ordered layers at the aqueous glycine-mineral interface. The adsorption process on the mineral surface is site competitive as both the glycine and water molecules show equal affinity toward surface Ca2+ cations. The glycine molecules interact directly with the FAp (100) surface, where one of their carboxylate groups coordinates with the surface Ca2+ cations. From the surface structure refinement, atomic positions of one glycine and four water molecules per unit cell were determined, along with the atomic relaxations in the FAp (100) surface. Molecular dynamic simulations were used to determine the long-range order of the adsorbate layers by investigating the hydrogen bonds.

Structure of interfacial water on fluorapatite (100) surface.

The structure relaxation mechanism of the fluorapatite (100) surface under completely hydrated ambient conditions has been investigated with the grazing incidence X-ray diffraction (GIXRD) technique. Detailed information on lateral as well as perpendicular ordering corresponding to the water molecules and atomic relaxations of the (100) surface of fluorapatite (FAp) crystal was obtained from the experimental analysis of the CTR intensities. Two laterally ordered water layers are present at the water/mineral interface. The first layer consists of four water molecules located at 1.6(1) A above the relaxed fluorapatite (100) surface while the second shows the presence of only two water molecules at a distance of 3.18(10) A from the mineral surface. Thus, the first layer water molecules complete the truncated coordination sites of the topmost surface Ca atoms, while the second water layer molecules remain bonded by means of H-bonding to the first layer molecules and the surface phosphate groups. Molecular mechanics simulations using force field techniques are in good agreement with this general structural behavior determined from the experiment.