Tedd E. Lister

Morphological and Microstructural Stability of Boron‐Doped Diamond Thin Film Electrodes in an Acidic Chloride Medium at High Anodic Current Densities

Boron-doped diamond thin films have been examined before and after high-current-density electrolysis to investigate the morphological and microstructural stability of this new electrode material. The diamond thin films were used to generate chlorine from a solution of 1.0 M HNO{sub 3} + 2.0 M NaCl at current densities of 0.05 and 0.5 A/cm{sup 2} for times up to 20 h. Comparative studies were made using common graphitic electrodes including highly oriented pyrolytic graphite, glassy carbon, and Grafoil{reg_sign}. The electrodes were characterized using four-point probe resistivity measurements, atomic force microscopy, scanning electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, and cyclic voltammetry. In all cases, no severe morphological or microstructural damage (i.e., corrosion) was observed on films exposed to the highest current density. There were surface compositional changes in the forms of oxygenation and non-diamond carbon impurity etching that produced an increase in the reaction overpotential. Specifically, the overpotential was supposed to result from a combination of decreased surface conductivity due to the formation of carbon-oxygen functional groups and loss of kinetically active redox sites due to the oxidative etching of nondiamond carbon impurities.

Se adlattices formed on Au(100), studies by LEED, AES, STM and electrochemistry

Abstract Ordered selenium atomic layers have been formed electrochemically on Au(100) at a series of coverages. Cyclic voltammetry and coulometry were used to study the deposition process, and to determine the corresponding coverages of a number of Se structures. Structures, with Se coverages of 0.25, 0.33, 0.5, and 0.89 monolayers, were identified using ultra high vacuum — electrochemical techniques as well as scanning tunneling microscopy. The corresponding unit cells of those structures were: p(2 × 2), (2 × √10), c(2 × 2), and a mostly (3 × √10), composed of close-packed Se 8 rings. Pit formation, associated with the formation of the densely packed Se 8 ring structure, was observed. They are reminiscent of pits observed in self-assembled monolayers of alkane thiols on Au surfaces. The pits disappeared as the structure, composed of Se rings, was converted to lower coverage structures, such as the 0.25 monolayer p(2 × 2), via anodic stripping. Se atomic layers were formed electrochemically in three ways: direct reduction from a HSeO − 3 solution; anodic stripping of previously formed bulk Se; or cathodic stripping of previously formed bulk Se. All three methods resulted in equivalent atomic layer structures on the Au(100) surface, but with some variation in the homogeneity and distribution of particular structures.

Formation of the first monolayer of CdSe on Au(111) by electrochemical ALE

Abstract Electrochemical ALE (ECALE), the electrochemical analog of atomic layer epitaxy (ALE), is being developed. Central to ALE is the formation of atomic layers of the elements using surface limited reactions, in a cycle. In electrochemistry another name for a surface limited electrodeposition reaction is underpotential deposition (UPD). In ECALE, compound thin films are formed an atomic layer at a time by using UPD, in a cycle. In the study presented here the formation of the first monolayer of CdSe on Au(111) is described. The first step in the ECALE cycle is the formation of an atomic layer of Se via UPD. UPD of Se resulted in a surface composed of domains of domains of two structures: a (√3 × √3)R30° structure at 1 3 coverage, and a close packed layer of square eight membered Se rings. The ring structure consists of three domains, each rotated by 120°. Formation of a single structure, homogeneously covering the whole surface proved difficult due to the kinetics of the UPD process. Subsequent deposition of a Cd atomic layer resulted in formation of a CdSe structure with a (3 × 3) unit cell, and 4 9 coverage of both Cd and Se. A structure proposed to account for the unit cell and coverages, consists of a slice out of the bulk wurtzite structure for CdSe laid onto the Au(111) substrate, where twice the surface lattice constant for CdSe matches up with three times the Au interatomic spacing.

A Comparison of Atomic Layers Formed by Electrodeposition of Selenium and Tellurium Scanning Tunneling Microscopy Studies on Au(100) and Au(111)

Structures formed by the electrodeposition of atomic layers of chalcogenides Se and Te, on Au(100) and Au(111), are described and compared. Each element, on each surface, forms a low coverage structure, consisting of atoms packed simply in high coordinate sites at distances just above their van der Waals diameter. As coverages are increased above this level, structures composed of chalcogenide atom chains or rings are formed. It is proposed that these chains or rings have significant molecular character, involving orbital overlap of adjacent chalcogenide atoms. Mechanisms are described to account for the formation of these chains and rings. Discussion is also presented concerning the appearance of triangular phase boundaries for both chalcogenides on Au(111). In the case of Se, isolated triangles, about 4-6 nm on a side are distributed across the surface, whereas a network of triangular phase boundaries is observed in the deposition of Te. The triangular phase boundaries in Se appear to result from the nucleation of domains in different threefold sites on Au(111). For Te, however, it is proposed that the triangular domains and phase boundaries are the result of Te atoms being too large to form an extended (√3X√3)R30° structure.

The effect of localized electric fields on the detection of dissolved sulfur species from Type 304 stainless steel using scanning electrochemical microscopy

Abstract This paper discusses a method for imaging localized sulfur concentrations dissolved from sulfide inclusions in Type 304 stainless steels. The method involves amperometric detection using the I − /I 3 − redox couple as a mediator for sulfide oxidation. The microelectrode current for I − oxidation is amplified by chemical reduction of I 3 − to I − by dissolved thiosulfate and/or H 2 S. While galvanostatically inducing corrosion, an unexpected current inversion at the microelectrode was observed at higher sample current densities in close proximity to the area where sulfur was detected at lower sample current. In the article first describing this method, J. Electrochem. Soc. 147 (2000) 4120 did not observe this feature. It was found that the inverted feature resulted from the localized increase in the electric field at the corrosion site, shifting the potential of the microelectrode when positioned over the site. The electric field measurement is analogous to measurements performed using a scanning vibrating electrode technique (SVET). This paper presents the first known combination of localized electric fields and chemical detection for determining localized electrochemical activity at a corroding surface.

Preparation of Au single crystals for studies of the ECALE deposition of CdTe

Abstract A sequence of questions concerning the preparation of atomically flat Au surfaces is addressed. In addition, the formation of a 1 2 monolayer Te adlayer on a Au(100) is discussed. The 1 2 coverage Au(100)-Te surface is desired as a first step in the electrodeposition of CdTe by the ECALE methodology. The use of oxidation/reduction cycles (ORCs) to clean the Au(100) surface is shown to be counter productive, if atomically flat planes are also desired. Roughening results from the ORCs as the surface reconstructs at low potentials, then unreconstructs at high potentials. The reconstructed surface contains excess surface atoms, such that upon lifting of the reconstruction, monoatomically high Au islands are formed on the surface. It is also shown that treatment in a mM I− solution results in the formation of ordered iodine adlattices on each of the low index planes. In addition, electrochemical annealing is facilitated, resulting in the removal of previously formed Au islands, and formation of large, atomically flat terraces. Te can be deposited directly on these I coated surfaces, displacing the I and forming thin films of bulk Te. The excess, bulk, Te can then be removed either reductively or oxidatively, to form ordered atomic layers of Te with coverages near 1 2 .

Electrochemical formation of Se atomic layers on Au(100)

Studies of the electrodeposition of atomic layers of Se have been prompted by their importance in the formation of II–VI compound semiconductors by electrochemical atomic layer epitaxy (ECALE). ECALE is a method where a compound is formed by the sequential, alternated, electrodeposition of atomic layers of the constituent elements making up a compound. It has been shown that the structure of the first atomic layer is generally the most critical. The present article describes three different methods for forming that first atomic layer. Studies of the structures resulting from those methods have revealed four distinctly different Se structures, all of which can be considered atomic layers, as none is more than a single‐Se atom thick. Graphs of coverage versus potential have been obtained for each of the methods, and show distinct plateaus over as much as 0.15 V, indicating the stability of some of the structures. The sequence of structures first involves the formation of a 1/4 coverage (2×2), followed by a ...

Scanning Electrochemical Microscopy Study of Corrosion Dynamics on Type 304 Stainless Steel

Scanning electrochemical microscopy was utilized to study dynamics of corrosion on type 304 SS. The I - /I 3 - redox couple was used in the substrate generation/tip collection mode to determine active sites. Scanning the sample sequentially revealed that the electrochemical activity (where I - 3 was detected) was localized and dynamic in nature. Subsequent microscopy indicated pitting occurred at many of the sites where electrochemical activity was detected. Currently, the technique is limited by the time required to raster a single probe through the scan area. In future studies, parallel (multielectrode) image acquisition is proposed to address this issue.

Corrosion of thermal spray hastelloy C-22 coatings in dilute HCl

The microstructure and corrosion behavior of Hastelloy C-22 coatings produced using the high velocity oxygen fuel (HVOF) method have been determined and related to in-flight measurements of the particle velocity and temperature. Average particle temperatures ranged from 1280–1450 °C and velocities ranged from 565–640 ms−1. All of the coatings were greater than 98% of theoretical density and exhibited passivating behavior in 0.1 M HCl during cyclic potentiodynamic polarization testing. The passive current density was somewhat higher compared with wrought C-22 alloy and an active-passive peak attributed to the formation of a Cr-rich surface layer was observed. Resistance of corrosion and deposition efficiency improved as the particle temperature decreased. There was little effect of particle velocity on the corrosion behavior over the range of deposition conditions examined. Our results suggest that feedback control based on measurement of the particle temperature can be used to process coatings with optimum properties.

Development and testing of an advanced neutron-absorbing gadolinium alloy for spent nuclear fuel storage

The U.S. Department of Energy requires nuclear criticality control measures for storage of its highly enriched spent nuclear fuel. A new alloy based on the Ni-Cr-Mo alloy system with a gadolinium addition has been developed. Gadolinium has been chosen as the neutron absorption alloying element because of its high thermal neutron absorption cross section. The metallurgical development, mechanical and physical properties, thermal neutron absorption properties, and accelerated corrosion-testing performance of this Ni-Cr-Mo-Gd alloy is described. A brief comparison is also included of the corrosion performance of this alloy as compared to borated stainless steel, which is commonly used as a neutron-absorbing, structural alloy.