Gary L. Borges

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.

An in-situ electrochemical quartz crystal microbalance study of the underpotential deposition of copper on Au(111) electrodes

The combination of the quartz crystal microbalance (QCM) with electrochemical methods has made possi- ble the in situ measurement of minute mass changes that may accompany electrode processes, such as elec- trodeposition, film growth, oxide formation, ion update into polymer films, and ionic adsorption [ll. While the electrochemical QCM provides valuable coverage in- formation which is often difficult to obtain by other means and complements other surface-sensitive in situ probes of smooth electrodes such as infrared (IR) spectroelectrochemistry, X-ray absorption and scatter- ing, scanning tunneling microscopy @TM), and non- linear optical spectroscopy [2], a significant limitation has been the inability to apply the QCM to ordered single crystal surfaces. We report here the first preparation of a highly ordered Au(lll) electrode on a quartz crystal. We also report results obtained using this electrode in an elec- trochemical QCM in conjunction with cyclic voltamme- try and coulometry to investigate the underpotential deposition (UPD) of copper onto the Au(ll1) surface. Even though copper UPD on gold has been studied using a QCM on polycrystalline surfaces [3,4] and by many other in situ interfacial techniques such as sur- face extended X-ray absorption fine structure (SEXAFS) [5], STM [6,7], and IR spectroelectrochem- istry [S], etc. on both single crystal and polycrystalline surfaces, there is still controversy as to the structure and composition of the adlayer at various stages in the UPD process. The QCMs consisted of 2.5 cm diameter AT-cut quartz crystal disks (Valpey-Fisher), which were pol- ished to an optical finish on both sides. Gold elec- trodes were vapor deposited by a modification of, the procedure described previously [3]. First, ca. 50 A of chromium metal (99.99%, Aldrich) was evaporated onto one side of the crystal to enhance the adhesion of gold. Then 2000 A of gold (99.999%, Aldrich) was evapo- rated onto the Tame side. These evaporations were done at room temperature in a vacuum ff 2

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.

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.

In Situ Surface EXAFS Study of an Underpotentially Deposited Silver Monolayer on Gold (111)

Fluorescence detected surface extended x-ray absorption fine structure (EXAFS) has been used to probe the structure of underpotentially deposited Ag on a Au(111) surface from an aqueous solution using sodium perchlorate as the electrolyte. Silver is shown to be fully reduced on the surface with observed bond distances of 2.88±0.025 A and 2.91±0.025 A for silver-silver and silver-gold near neighbors, respectively. Data is obtained with the polarization of the incident x-ray beam both parallel and perpendicular to the electrode surface

Grazing Incidence X‐Ray and Electrochemical Study of Thin Film Copper(111) on Mica

The preparation of evaporated thin films of largely single-crystalline copper on mica substrates is reported. Most of the exposed copper surface has (111) orientation, as established by grazing incidence x-ray scattering and electrochemical studies. The (111) surface domains are on the order of 300 {angstrom} in diameter and have a mosaic spread of ca. 0.1{degrees}. Underpotential deposition (UPD) shifts for Pb and Tl on these Cu(111) films are approximately 150 300 mV, respectively. In this paper Tl is shown to form a UPD bilayer on the substrate. These epitaxial Cu films can be utilized in electrochemical surface/interface studies.

ELECTROCHEMICALLY ADSORBED PB ON AG (111) STUDIED WITH GRAZING INCIDENCE X-RAY SCATTERING

Grazing-incidence x-ray scattering studies of the evolution of electrochemically deposited layers of lead on silver (111) as a function of applied electrochemical potential are presented. Measurements were made with the adsorbed layers in contact with solution in a specially designed sample cell. The observed lead structures are a function of the applied potential and range from an incommensurate monolayer, resulting from underpotential deposition, to randomly oriented polycrystalline bulk lead, resulting from lower deposition potentials. These early experiments demonstrate the ability of in situ x-ray diffraction measurements to determine structures associated with electrochemical deposition. 6 refs., 4 figs.