Lorena H. Klein

In situ STM study of the duplex passive films formed on Cu(111) and Cu(001) in 0.1 M NaOH

In situ electrochemical scanning tunneling microscopy (ECSTM) investigations of the anodic Cu(I)/Cu(II) duplex passive layers grown on Cu(1 1 1) and Cu(0 0 1) in 0.1 M NaOH are reported. The outer Cu(II) part of the duplex film formed on both substrates is crystalline with a terrace and step topography. The observed lattices are consistent with a bulk-like terminated CuO(0 0 1) surface on both substrates. This common crystallographic orientation is explained by the hydroxylation of the otherwise polar and unstable oxide surface at the passive film/electrolyte interface. The epitaxy of the oxide layers is governed by the parallel alignment of the close packed directions of the CuO outer layers and Cu2O inner layers on both substrates. A granular and amorphous layer covering the crystalline CuO(0 0 1) oxide has been observed on Cu(0 0 1) but not on Cu(1 1 1). It is assigned to a film of copper hydroxide corrosion products formed by a dissolution–precipitation mechanism. Its absence on the passivated Cu(1 1 1) surface is explained by the higher stability of the Cu2O(1 1 1) precursor oxide formed on this substrate in the initial stages of growth of the duplex passive film, resulting in a lower amount of dissolved copper.

In situ STM study of the anodic oxidation of Cu(0 0 1) in 0.1 M NaOH

In situ electrochemical scanning tunneling microscopy measurements of the anodic oxidation of Cu(0 0 1) in 0.1 M NaOH are reported. Adsorption-induced surface reconstruction is observed in the underpotential range of oxidation with the formation of dimers of superimposed Cu atoms ejected from the substrate and stabilized by adsorbed OH groups, presumably in bridging positions. The reconstruction causes the reorientation of the substrate step edges and the formation of holes and ad-islands of monoatomic height. The dimers of superimposed Cu atoms are alternatively aligned along the � 100 � directions to form zig-zag arrangements. Long range ordering is observed in areas of limited lateral extension with c(2/6) and c(6/2) domains. In the potential range of Cu(I) oxide formation, a facetted Cu2O layer grows with a Cu2O(0 0 1) /[1 ¯ 1 0] / jj Cu(0 0 1)[1 0 0] epitaxial relationship. The 458 rotation between the close-packed directions of the oxide lattice and metal lattice results from the orientation of the dimers of superimposed Cu atoms in the precursor adsorbed OH layer. The surface of the oxide layer is facetted due to a tilt of � /3% between oxide and metal lattices. Its (0 0 1) terraces have an identical chemical termination and are presumably hydroxylated. # 2003 Elsevier Science B.V. All rights reserved.

In situ STM study of the effect of chlorides on the initial stages of anodic oxidation of Cu(111) in alkaline solutions

Abstract In situ electrochemical scanning tunneling microscopy (STM) has been applied to study the mechanisms of growth of passive layers on Cu(111) in NaOH solutions in the presence of chlorides. For [Cl−]/[OH−]=0.01, the same ordered precursor phase of adsorbed OH is observed in the underpotential region of oxidation as in Cl−-free solutions. Atomically resolved images reveal the structure of the reconstructed topmost metal plane and the threefold hollow adsorption site of the hydroxide. The induced reconstruction causes the ejection of Cu atoms that contribute to the observed lateral growth of the terraces and to the formation of 2D Cu ad-islands in the final stages of the adsorption process. For [Cl−]/[OH−]=0.1, threadlike nanostructures resulting from the reaction of the ejected Cu atoms with chlorides are formed before agglomeration with the 2D Cu ad-islands formed in the final stage of the hydroxide adsorption process. For [Cl−]/[OH−]=10, the step edges, which are normally the preferential sites of the reaction with hydroxide, are blocked by the formation of non-ordered surface chloride complexes. Hydroxide adsorption still predominates the surface reaction on the terraces but the 2D ad-islands form immediately due to the blocking of the step edges. In the potential range of Cu(I) oxide formation, crystalline Cu(I) oxide layers are formed with a high density of steps and (111) terraces. Their step edges are rougher in the presence of chlorides which indicates a Cl−-enhanced localized dissolution reaction of the oxide layers at step edges.

In Situ STM Study of the Effect of Chloride on Passive Film on Nickel in Alkaline Solution

In situ electrochemical scanning tunneling microscopy (STM) measurements on the growth and structure of the passive film on Ni(111) in 0.1 M NaOH + xM NaCl (x = 0, 0.05, 0.1, and 1 M) aqueous solutions are reported. At the onset of the active-passive transition, the formation of the (2 × 2) islands, assigned to the locally ordered coadsorption of hydroxide species and water molecules on the Ni(lll) unreconstructed surface, is retarded ([Cl - ]/[OH - ] ≤ 1) or blocked ([Cl - ]/[OH - ] = 10) by a competitive adsorption mechanism between OH - and Cl - . A competitive mechanism, at step edges, of metal dissolution and nucleation of a 2D passive film is also observed. For ([Cl - ]/[OH - ] ≥ 1), the nucleation of the passive film at the step edges is blocked at E = -500 mV/SHE (standard hydrogen electrode). The rate of retraction of the step edges is not significantly enhanced by the chlorides, but the dissolution is sustained. Increasing the potential unblocks nucleation but leads to the formation of 3D nuclei. At E ≥ -175 mV/SHE, a crystalline passive film develops for [Cl - ]/[OH - ] ≤ 0.5. At higher chloride concentration, the crystallization is blocked, and clusters of nanograins (10 ± 5 nm) are formed. Further growth of the grains by coalescence is blocked. Localized attacks of the substrate are developed by preferential dissolution at the boundaries between the grain clusters.

In Situ STM Study of the Initial Stages of Anodic Oxidation of Cu(111) in the Presence of Sulfates

In situ scanning tunneling microscopy (STM) has been applied to study adsorption and oxide formation on Cu(111) in weakly alkaline, neutral, and acidic sulfate-containing solutions. In 5 × 10 4 M NaOH + 5 × 10 3 M Na 2 SO 4 (pH ∼ 10.5), a hexagonally structured OH layer is formed at potentials well below oxide formation despite the tenfold larger sulfate concentration showing a stronger interaction of OH with the Cu surface than SO 2 4. In 5 X 10 3 M Na 2 SO 4 (pH ∼ 7), a partially structured and highly mobile adlayer is formed at potentials quite near the onset of oxide formation with coexisting (3 × 3)-ordered domains and nonordered domains. This is assigned to the competitive adsorption of water molecules with sulfate anions. In contrast, a highly ordered and stable (√3 x √7) adsorbed layer is formed in 5 x 10 - 3 M H 2 SO 4 . It is assigned to the coadsorption of sulfate anions and cations (H 3 O + or H + ), the latter compensating the repulsive forces between sulfate anions and thus stabilizing the adlayer. The anodic Cu(1) oxide is (111)-oriented and has a faceted surface in solutions of alkaline, weakly alkaline. and neutral pH.

Aging-induced chemical and morphological modifications of thin film iron oxide electrodes for lithium-ion batteries.

Spectroscopic (XPS, ToF-SIMS) and microscopic (SEM, AFM) analytical methods have been applied to iron oxide (∼Fe2O3) using a thin film approach to bring new insight into the aging mechanisms of conversion-type anode materials for lithium-ion batteries. The results show that repeated lithiation/delithiation causes both chemical and morphological modifications affecting the electrochemical performance. The SEI layer formed by reductive decomposition of the electrolyte remains stable in composition (mostly Li2CO3) but irreversibly thickens upon multicycling. Irreversible swelling of the material accompanied by penetration of the SEI layer and accumulation of non-deconverted material in the bulk of the oxide thin film occurs upon repeated conversion/deconversion. After initial pulverization of the thin film microstructure, grain growth and aggregation are promoted by multicycling. This leads to capacity increase in the first few cycles, but upon further cycling volume expansion and accumulation of non-deconverted material lead to deterioration of the electrode performances.

ToF-SIMS Imaging Study of the Early Stages of Corrosion in Al-Cu Thin Films

The pitting corrosion of Al-Cu thin film alloys was investigated using samples that were heat treated in air to form through-thickness Al 2 Cu particles within an Al-0.5% Cu matrix. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) analysis revealed Cu-rich regions 250―800 nm in lateral extent near the metal/oxide interface. Following exposure that generated pitting corrosion, secondary electron, secondary ion, and AFM images showed pits with size and density similar to those of the Cu-rich regions. The role of the Cu-rich regions is addressed.

Preparation and characterization of an electronically conductive and chemically modified ultrafiltration type membrane

Abstract We have prepared, for the first time, an electronically conductive polyethersulfone ultrafiltration membrane, noted as PS, by depositing an ultrathin gold coating. This conductive membrane has been chemically modified by electrodepositing a polymer-based nickel tetrasulfonated phthalocyanine film (NiTSPc) covered by Nafion ® . Atomic force microscopy (AFM) and membrane potential measurement experiments were carried out to characterize the morphology and the transport properties of the new PS membrane. AFM measurements showed that the topology of the surface pores of the PS membrane was modified by the deposition of Au, NiTSPc and Nafion ® layers and that the chemical modifications of the membrane do not obstruct the access to the inner pores. Li + transport number determination have corroborated AFM results and showed that the Li + transport was facilitated by electrostatic effect induced by the negative charge densities of NiTSPc and NiTSPc/Nafion ® coatings.

Local Degradation Mechanisms by Tarnishing of Protected Silver Mirror Layers Studied by Combined Surface Analysis

In this work, we addressed the local degradation mechanisms limiting the prelaunch environmental durability of thin-layered silver stacks for demanding space mirror applications. Local initiation and propagation of tarnishing were studied by combined surface and interface analysis on model stack samples consisting of thin silver layers supported on lightweight SiC substrates and protected by thin SiO2 overcoats, deposited by cathodic magnetron sputtering and submitted to accelerated aging in gaseous H2S. The results show that tarnishing is locally initiated by the formation of Ag2S columns erupting above the stack surface. Ag2S growth is promoted at high aspect ratio defects (surface pores) of the SiC substrate as a result of an imperfect protection by the SiO2 overcoat. Channels most likely connect the silver layer to its environment through the protection layer, which enables local H2S entry and Ag2S growth. The silver sulfide columns grow in number and size eventually leading to coalescence with increasing H2S exposure. In more advanced stages, tarnishing slows down owing to saturation of all pre-existing imperfectly protected sites of preferential sulfidation. However, it progresses radially at the basis of the Ag2S columns underneath the protection layer, consuming the metallic silver layer and deteriorating the protecting overcoat.

Nanoscale Intergranular Corrosion and Relation with Grain Boundary Character as Studied In Situ on Copper

The initiation of intergranular corrosion at various types of grain boundaries (GBs) was studied at the nanometer scale on microcrystalline copper in 1 mM HCl aqueous solution. In situ Electrochemical Scanning Tunneling Microscopy (ECSTM) and Electron Back-Scatter Diffraction analysis of the same local microstructural region were combined using an innovative methodology including micro marking performed with the STM tip. The results demonstrate that electrochemically-induced intergranular dissolution, at the surface termination of GBs, is dependent on the grain boundary character. It is found that random high angle boundaries as well as sigma9 coincidence site lattice (CSL) boundaries are susceptible to nanoscale initiation of intergranular corrosion while for sigma3 CSL boundaries the behavior is dependent on the deviation angle of the GB plane from the exact orientation. For the sigma3 twins, a transition from resistance to susceptibility occurs between 1{\deg} and 1.7{\deg} of deviation as a result of the increase of the density of steps (i.e. misorientation dislocations) in the coincidence boundary plane. The work emphasizes the precision needed in the design of the grain boundary network in applications where intergranular corrosion or its initiation must be controlled at the nanoscale.