Owen R. Melroy

Underpotential metal deposition on gold, monitored in situ with a quartz microbalance

Abstract The underpotential deposition (UPD) of Pb, Bi, Cu, and Cd has been examined on polycrystalline Au in 0.1 M HClO4. The Au film electrode was attached to an oscillating quartz crystal microbalance (QCM) mounted in an electrochemical cell. The coverage of the UPD layer on the Au surface is calculated from the frequency change of the QCM as a function of potential. Coverages are in good agreement with previous estimates obtained using other techniques. The electrosorption valencies (γ) have been calculated from the charge to coverage ratios obtained from the QCM. Values for the four metals, Pb (γ = 2.0), Bi (γ = 2.7), Cu (γ =1.4). and Cd (γ = 1.6 to 2.0), are similar to those previously reported. These experiments indicate that the QCM is a quantitative tool for examining monolayer and sub-monolayer metal deposits.

Ion flux during electrochemical charging of Prussian Blue films

Abstract Thin (approximately 100 nm) films of Prussian Blue were electrochemically deposited onto mass-sensitive oscillating quartz-crystal electrodes. Changes in electrode mass during the electrodeposition process indicated that the as deposited films were highly hydrated. Mass changes upon electrochemical reduction and oxidation of the Prussian Blue films in the nitrate salts of K + , Rb + , and Cs + at pH 4, were measured. Some weight gain occurred upon the original reduction and reoxidation, indicating that an irreversible structural reorganization occurred. Subsequent electrochemical cycling produced a reversible mass gain upon reduction, and a reversible mass loss upon oxidation, indicating that alkali metal cations enter and exit the film upon reduction and oxidation, respectively. The mass/charge behavior of the films was studied in 0.5 M RbNO 3 , over the pH range 2.7–6.4. The pH dependence indicated that considerable proton expulsion can occur upon oxidation of the Prussian Blue, and some proton inclusion can accompany reduction. The film mass losses upon oxidation in 0.5 M RbNO 3 and 0.5 M RbOOCCH 3 , pH 4, were virtually indistinguishable, implying that the supporting electrolyte anions played little role in the electrochemistry in these media.

Distribution of water molecules at Ag(111)/electrolyte interface as studied with surface X-ray scattering

The spatial distribution of water molecules at solid-electrolyte interfaces has received extensive theoretical study, due to the importance of this interface in electrochemistry and other sciences. Such studies suggest that adjacent to the interface water is arranged in several layers, that the molecular arrangements in the inner layer is similar to bulk water, and that the inner-layer molecules have an oxygen-up (oxygen-down) average orientation for negative (positive) electrode charge (or, equivalently, potential). However, little of this has been verified by experimental measurements. In this paper we report surface X-ray scattering measurements of the water distribution perpendicular to a Ag(111)-electrolyte interface in 0.1M NaF at two potentials: +0.52 and −0.23 V from the potential of zero charge (PZC) on the electrode. We find that, first, the water is ordered in layers extending about three molecular diameters from the electrode. Second, the extent of ordering and the distance between the electrode and first water layer depend on potential, the latter being consistent with an oxygen-up (oxygen-down) average molecular orientation for negative (positive) electrode potential. Third, the inner water layer contains 1.55 × 1015 (at −0.23 V) and 2.6 × 1015 (at +0.52 V) water molecules per cm−2, remarkably more than expected from the bulk water density (i.e., ∼ 1.15 × 1015cm−2). Such a large compression shows that the molecular arrangements in the inner layer are significantly different from bulk, which has not been anticipated in current models of charged, aqueous interfaces. We give a qualitative explanation of this large density as resulting from the strong electric field at the charged Ag(111) electrode and present a tentative model of the molecular arrangements.

Study of underpotentially deposited copper on gold by fluorescence detected surface EXAFS

Using grazing incidence geometry and fluorescence detection, surface EXAFS of a monolayer of underpotentially deposited copper on epitaxially deposited gold (111) on mica was observed. Both Cu–Au and Cu–O scattering are observed. The results are interpreted in terms of models in which the Cu–O distance is 2.08±0.03 A and the Cu–Au distance is 2.5±0.06 A. The copper and three gold atoms at the metallic surface form an elongated tetrahedron, with the oxygen on top of the copper. Two plausible models for the oxygen arrangement are proposed, one involving only one oxygen, the other with a sulfate ion adsorbed. This second model yields a slightly better fit of the data.

Observation of morphological relaxation of copper and silver electrodes in solution using a quartz microbalance

Abstract Oxidation of smooth copper and silver surfaces leads to roughening which persists when the oxide layers are reduced. Furthermore, the roughened surface is observed to relax on a time scale of hours when potentiostated at reductive potentials. Reconstruction was monitored in-situ by differential capacitance and a quartz microbalance oscillating in solution. Scanning electron microscopy (SEM) gave supporting evidence. Estimates of the surface roughness of the oxide layers and the bare metal were obtained from the microbalance data.

A study of the electrosorption of bromide and iodide ions on gold using the quartz crystal microbalance

Abstract The adsorption of I− and Br− have been examined on polycrystalline gold in 50 mM NaClO4. The mass of the adsorbed layer was measured in-situ with a quartz crystal microbalance using one of the gold electrodes on the oscillator as the working electrode in an electrochemical cell. In the concentration range studied, 0.1 mM to 10 mM, both iodide and bromide were found to adsorb to full monolayer coverage. The electrosorption valencies γ, were calculated from plots of charge versus coverage and were found to be 1.01 ±0.05 and 0.39 ±0.03 for iodide and bromide respectively.

An in-situ grazing incidence X-ray scattering study of the initial stages of electrochemical growth of lead on silver (111)

Abstract The potential dependent structure of underpotentially deposited lead on silver (111) and the initial stages of bulk lead deposition on the ad-layer have been studied using grazing incidence X-ray scattering. Measurements were made in-situ and under potential control. The close packed triangular lattice of lead formed by the underpotential deposition (at full monolayer coverage) is compressed 1.4% relative to bulk lead. This compressive strain increases linearly with applied potential until the onset of bulk deposition where the ad-layer is compressed 2.8%. Bulk lead is not deposited epitaxially on this template because of the large compressive strain. Instead, it grows as islands that have (111) texture but are randomly oriented in the plane of the substrate. After the deposition of approximately five equivalent monolayers of bulk lead, the initial ad-layer appears to reconstruct.

Structure of metal-electrolyte interfaces: Copper on gold(111), water on silver(111)

Two examples are presented of the application of in situ structure and composition sensitive techniques to elucidating the nature of the electrode-electrolyte interface. One case is the underpotentially deposited submonolayer of Cu on Au(111). Despite extensive electrochemical, LEED, RHEED, STM, AFM, and EXAFS investigations, the structure of the copper adlayer is still controversial. In situ surface EXAFS, surface X-ray scattering, and quartz crystal microbalance (QCM) data lead to the conclusion that the adlayer adopts a (√3 × √3)R30 ° hexagonal honeycomb arrangement of Cu atoms in which the Cu coverage is 23 and the sulfate coverage 13. The other case is the structure of water at the Ag(111)-aqueous sodium fluoride interface. Analysis of X-ray scattering shows that the interfacial water molecules exhibit potential dependent layering and reorientation and a markedly increased density (twice bulk water) in the inner layer.

The quartz resonator: electrochemical applications

Since the discovery that the oscillations of resonating quartz crystals can be sustained in a liquid environment, such crystals have quickly found use as a sensitive microbalance in electrochemistry, making possible in situ measurements of mass changes at the electrochemical interface. The early contributions of the IBM Almaden Research Center to this exciting field of development are sketched. The principles of operation are detailed, with emphasis on an intuitive description to permit considerations of new applications. Mass density changes of the order of 10 nanograms per square centimeter (ng/cm2) are routinely detectable as changes in the resonant frequency of about a hertz. The mass density of a monolayer of material ranges from a few tens of ng/cm2 for polymeric materials to a few hundreds of ng/cm2 for metals. Detailed analysis of the electrical behavior of the resonator in liquid media shows that the resonant frequency, the quality factor of the resonance, and the admittance at resonance are all sensitive to the viscoelastic properties of the contacting liquid, having implications in the study of the behavior of non-Newtonian fluids, including polymeric films.

In-situ grazing incidence X-ray diffraction study of electrochemically deposited Pb monolayers on Ag(111)

Abstract We report here the first in situ X-ray diffraction measurements from a monolayer adsorbed at a metal/liquid interface (lead electrochemically deposited on silver (111)). The lead was found to order into a close-packed hexagonal structure, compressed 1.2% from bulk lead. The first order diffraction peak was 0.037 A −1 broad in the radial and azimuthal directions, indicating that even in an aqueous environment the Pb monolayer forms a well-ordered two-dimensional solid. A rotational epitaxy angle (angle between the Pb and Ag surface lattices) of 4.4° was observed.