Joseph G. Gordon

The oscillation frequency of a quartz resonator in contact with liquid

A simple relationship is derived which expresses the change in oscillation frequency of a quartz crystal in contact with a fluid in terms of material parameters of the fluid and the quartz. The relationship is Δf = −f320(ηLϱL/πμQϱQ)12, where f0 is the oscillation frequency of the free (dry) crystal, ηL and ϱL are the absolute viscosity and density of the liquid, respectively, and μQ and ϱQ are the elastic modulus and density of the quartz. This relation is obtained from a simple physical model which couples the shear wave in the quartz to a damped shear wave in the fluid. Quantitative comparisons with two test cases, aqueous solutions of glucose and ethanol at various concentrations, demonstrate the accuracy of this model.

Surface plasmon spectroscopy of organic monolayer assemblies

Abstract Because the surface plasmon resonance is a sensitive probe of metallic surfaces we have measured quantitatively the changes that occur in surface plasmon resonances from coatings of thin layers of cadmium arachidate of different but known thicknesses on silver films. An ATR (attenuated total reflection) experimental arrangement was used. The dispersion of the refractive index of the organic layer was derived by a least square fit of the measured reflectivity curves and it agrees well with other data obtained by different methods. The influence of a possible intermediate layer between the silver surface and the first organic monolayer due to the preparation process is also discussed.

Electrode/electrolyte interphase study using polarization modulated ftir reflection-absorption spectroscopy

Abstract Polarization modulated Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS) is applied to the studies of adsorption and oxidation of CO on a platinum electrode in 0.5 M sulfuric acid and of adsorption of cyanide on gold and silver electrodes in 0.5 M potassium sulfate. The absorption intensity of the CO on platinum electrode is ~ 4–5% while that of the CN − on silver and gold is 0.2–0.5%. The potential dependence of the vibrational spectra was observed for both systems. Oxidation of the linearly adsorbed CO layer proceeds by different mechanisms depending on whether CO was adsorbed at a potential in the double-layer region or in the hydrogen region, i.e. at the edges of the CO islands in the former case and randomly in the latter case, in which the bridged CO species plays an important role. The vibrational frequency of the linearly adsorbed CO changes linearly with potential at a rate of 30 cm −1 /volt, which is independent of anion specific adsorption. The origin of the shift is most reasonably explained by the first-order Stark effect. For Ag/CN − and Au/CN − systems, the surface cyanide species is assigned as linearly adsorbed CN . The anodic reaction products in the solution from cyanide ions and the electrode metals are also observed in the vibrational spectra. The bands due to surface species and those due to solution species are distinguished by measuring the spectra with s- and p-polariz.ed light.

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.

Surface plasmons as a probe of the electrochemical interface

Abstract Surface plasmons (SP) or surface electromagnetic waves were excited at the silver-aqueous electrolyte interface with visible light using an attenuated total reflection arrangement. SP resonances were observed as a function of potential in perchlorate and halide electrolytes. At fixed wavelength, the resonance shifts to smaller wave vector as the electrode potential becomes more positive. Various contributions to this shift will be discussed: changes in electron density at the metal surface, ion adsorption and changes in the optical properties of the ionic double layer. Halide ion adsorption shifts the reflectivity: potential curve to more negative potentials.

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.

Contribution of specifically adsorbed ions, water, and impurities to the surface enhanced Raman spectroscopy (SERS) of Ag electrodes

Surface enhanced Raman scattering (SERS) has been observed from silver electrodes for water (H2O and D2O) in the frequency region of the librational, bending, and stretching modes. Simultaneously, SERS has been observed for halide ions and some organic impurities. The appearance of SERS from water and halide ions under the circumstances of the experiment is attributed to the formation of surface complexes involving silver adatoms, halide ions, and water molecules.

Surface-enhanced Raman spectroscopy of electrochemically characterized interfaces; potential dependence of Raman spectra for thiocyanate at silver electrodes

Abstract Surface-Enhanced Raman Spectra have been obtained for thiocyanate anions at silver electrodes following an oxidation-reduction cycle (ORC) as a function of electrode potential and electrolyte composition and compared with the extent of thiocyanate adsorption determined under the same conditions from differential capacitance-potential data. A spectrograph equipped with an optical multichannel analyzer (OMA) detector and a scanning spectrometer were used to make the Raman measurements. Spectra were obtained over the frequency range 100–2200 cm −1 , where all three normal vibrations, CN stretching ( v CN , 2090–2120 cm −1 ), CS stretching ( v CN , 735 cm −1 ), NCS angle bending (δ NCS , 450 cm −1 ) occur, along with a low frequency vibration attributed to a metal-ligand stretching mode ( v ml , 200–215 cm −1 ) arising from a silver-sulfur surface bond. Both v CN and v ML decreased in frequency as well as intensity as the potential was made more negative in the region −100 to −700 mV versus Ag/AgCl for bulk thiocyanate concentrations of one millimolar and above, even though the thiocyanate surface concentration remained close to a monolayer throughout. By means of rapid time-resolved spectral measurements following potential steps using the OMA, the decrease of the intensity and frequency of the v CN mode with increasing negative electrode potential was separated into a rapid “reversible” component and a slower “irreversible” decay. The latter component is attributed to the decay of Raman-active sites associated with the dissipation of metastable silver clusters formed during the ORC, that are prevented from rearranging at more positive potentials due to the presence of surrounding anionic adsorbate.

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