An atomistic view of electrochemistry

One of the most important tasks of modern, physical electrochemistry is the development of an atomistic picture of the solid/liquid interface in order to provide the basis for a mechanistic understanding of electrochemical processes. Electrochemists seek answers to the same questions as their surface science colleagues (e.g., electronic and structure properties of surfaces and adlayers), but are faced with the fact that in electrochemistry the contact of the solid with a condensed phase, the electrolyte, makes life much more difficult. Nevertheless, electrochemists succeeded in the last 20 years to develop an electrochemical surface science by adopting experimental techniques and theoretical concepts from surface physicists.

This article describes the various routes electrochemists have used to obtain a detailed characterization of electrode surfaces in particular, and of the electrochemical interface in general. Success in physical electrochemistry is based on the development of non-traditional in situ methods to complement the classical, current- and voltage-based techniques. The former range from optical spectroscopies, linear and non-linear, to in situ X-ray diffraction and scanning tunneling microscopy. The current status of electrochemical surface science and its most important future goals are briefly addressed.     

Atomic description of elementary surface processes: diffusion and dynamics

A large fraction of processes which are at the foundation of our technological society involve physical and chemical properties of surfaces. Catalytic reactions and semiconductor devices production are two of the most important ones.

This paper describes a sample of some of the most relevant surface science experiments which have been recently performed, in order to understand elementary surface processes of model catalytic reactions and in semiconductor technology at the atomic level. The focus is on experiments performed with scanning tunneling microscopy and atomic force microscopy which have represented, in some cases, real breakthroughs in our understanding of these phenomena.

We then present an overview of possible experimental technique developments that can be foreseen for the future and that may give us a more in-depth understanding of the elementary processes which form the basis of important complex surface phenomena. Finally, some of the challenging tasks that lie ahead for surface scientists and the collateral opportunities are discussed.     

Filter switching device for dual-wavelength videoimaging

An inexpensive, dual-wavelength, videoimaging system that can be used for parallel observation of two fluorescent dyes is described. All four filters, two for excitation and two for emission, are placed on the same oscillating holder. Filters are coupled with a single dichroic mirror having two spectral windows. A coil driven by an electronic circuit connected to photosensors, which determine the position of the holder, moves the magnet that shifts the position of the filters. Since the filter holder is placed between two springs, it oscillates with the frequency of mechanical resonance. As a result the filter switching did not require much power and did not produce significant vibrations of the base. Switching frequencies up to 4.5 s^–1 were reached with the first experimental device. System performance was tested using phospholipid vesicles loaded with water-soluble and membrane dyes. It has been demonstrated that the device can be used successfully in experiments on membrane fusion with rhodamine- and calcein-labeled liposomes.     

Wetting, Bonding and Interfacial Energy of Nanocrystalline Metal Particles on Crystalline DCH Polymer and Amorphous Carbon Substrates

Experimental results concerning the interaction between a variety of nanocrystalline metals (gold, silver, nickel and chromium) and both crystalline polymer [poly-DCH (1,6-di (N-carbazolyl)-2,4-hexadiene)] and amorphous carbon substrates, are presented and analyzed. Attention is focused on aspects of the interaction that concern interfacial bonding, its correlation with the cohesive energies of the various metals and the energy of the interfaces. Experimental contributions include qualitative estimates of the magnitude of interfacial energies for the crystalline polymer/metal and amorphous carbon/metal interfaces and a direct measurement of the interfacial energies for gold and silver nanocrystals deposited on the amorphous carbon substrate. The sequence of interfacial energy values for the polymer/metal and amorphous carbon/metal systems is also determined. The interfacial energies for both the poly-DCH and amorphous carbon substrates decreases in the order silver, gold, nickel, and chromium, as expected from cohesive energy, melting point and surface energy data for these elements. The crystalline polymer/metal interfaces were examined for the presence of orientation relationships using selected area diffraction and optical diffractometry of high-resolution TEM images. No orientation relationships were found for any of the polymer/metal combinations spanning a large range of metal reactivities. Lack of atomic matching or some as yet unknown surface condition on the polymer may be responsible for this effect.     

Thin oxide films of silicon by high pressure oxidation

High pressure oxidation of silicon was performed at an oxidation temperature range of 650 to 800°C for thin oxide films with about 300 å thickness. The index of refraction was dependent on an oxidation temperature but independent of the oxidation pressure as 1.475 at 750°C. The dielectric breakdown strength of the oxide film was measured by the voltage ramping,method. The fixed oxide charge about 1.0 × 10¹¹ cm^?2 was also measured from the high frequency C-V measurement. The pulse scanning C-V measurement technique was used to measure the minority carrier generation rate in the depleted surface. The surface generation velocity was slightly dependent on the oxidation temperature and indicated that the fast surface states increased with decreasing oxidation temperature.     

Electron probe microanalysis of SiO2-coated minerals: Subsurface phases in quartz

Crystal fragments of biotite, garnet, and spinel were mounted together in one block which afterwards was sawn into several pieces; each piece was coated with SiO₂ of varying thickness, up to 7.9m. Electron probe microanalysis was carried out at various accelerating voltages of the major elements present, using the non-SiO₂-coated minerals as reference standard.Calibration curves, depth below the surface versus ratios of X-ray yields, have been established with the ultimate goal to use them for a (semi)-quantification of elements present in subsurface phases in quartz, a common host mineral in rocks.     

The Measurement of the Dispersion of Pd/C Catalysts

A 10 w/o Pd/C catalyst from Johnson Matthey has been examined by three different techniques: chemisorption, line broadening of X-ray diffraction, and TEM (transmission electron microscopy) to investigate the size of the palladium crystallites. The dispersion of this catalyst was determined to be 20%. Among these techniques, the X-ray diffraction was found to be the most convenient method. Through this method, the dispersion of Pd was found to decrease on catalyzing hydroxylamine reactions at 330 K. The deactivation of Pd/C for these reactions was correlated to the sintering of palladium. H₂ chemisorption at 373 K was found to be a good way to accurately measure the number of active palladium sites in the Pd/C catalysts.     

D-Proline capped gold nanoclusters for turn-on detection of serum Raltitrexed

Using D-proline (D-Pro) as the reducing agent and capper, D-Pro@AuNCs was rapidly constructed. Its fluorescence could be quenched by AuNPs. Due to the electrostatic interaction between anticancer drug Raltitrexed (RTX) and AuNPs induced fluorescence "turn-on" principle, the resultant fluorescent probe exhibited good selectivity and sensitivity for detecting RTX in rat serums.     

The TEM characterization of the lamellar structure of osteoporotic human trabecular bone

The lamellar structure of osteoporotic human trabecular bone was characterized experimentally by means of transmission electron microscopy (TEM). More specifically, the TEM was used to determine if trabecular bone exhibits similar lamellar structural motifs as cortical bone by analyzing unmineralized, mineralized and demineralized bone, and to study the influence of the osteocyte network on the lamellar structure of osteoporotic trabecular bone. Comparison with normal trabecular bone is included. This paper summarizes partial results of a larger study, which addressed the characterization of the hierarchical structure of normal versus osteoporotic human trabecular bone [Rubin, M.A., 2001. Multiscale characterization of the ultrastructure of trabecular bone in osteoporotic and normal humans and in two inbred strains of mice. MS Thesis, Georgia Institute of Technology.] at several structural scales.     

Vertebral cancellous bone turn-over: microcallus and bridges in backscatter electron microscopy

Backscatter electron microscopy (BSE) is a powerful technique for investigating cancellous bone structure. Its main function is to offer information regarding the degree of mineralization of the tissue within individual trabeculae.

To illustrate the qualitative information that can be drawn from BSE imaging technique, we present a study on human vertebral cancellous bone. This tissue is continuously remodeled through osteoclastic resorption and osteoblastic new bone apposition. It is thought that osteoclastic resorption pits are especially deleterious for vertebral bone architecture since they often perforate the thin trabeculae; the osteoblasts being unable to repair the gap. In addition, excessive stress may also disrupt the architecture in case of trabecular fracture or damage accumulation.

Waves of new bone formation were easy to identify in BSE. Often these waves were connecting both edges of a perforation and called bridges. Additionally, we present a few images of microcallus formations. A microcallus is described as a small mass of woven bone that generally repairs a trabecula. The microstructural aspects of different microcalluses are presented and discussed. Both bridges and microcallus should be considered as examples of the repair porcess since they obviously preserve the connectivity of the trabeculae. However, bridges were much more frequent than microcallus (396 vs 15). Both mechanisms probably illustrate the normal response to different local stimuli.