Arthur T. Hubbard

Study of platinum electrodes by means of electrochemistry and low-energy electron diffraction: Part II. Comparison of the electrochemical activity of Pt(100) and Pt(111) surfaces

Abstract Low-energy electron diffraction patterns were obtained for Pt(100), Pt(111) and polycrystalline electrodes before and after exposure to aqueous 1 M H 2 SO 4 . Linear potential scan voltammograms were recorded. The results demonstrate that one of the principal peaks in the hydrogen region of the current-potential curves of polycrystalline Pt is assignable to Pt(100) and the other to Pt(111). The maximum amount of chemisorbed hydrogen corresponds to one hydrogen atom per surface Pt atom. The Pt(100)[1×1], Pt(111) and polycrystalline surfaces appear to withstand prolonged voltammetric characterization at potentials between −0.2 and 1.2 V vs. a calomel reference. Variation of the voltammetric characteristics of hydrogen chemisorption with changes in the nature of the supporting electrolyte anion are described.

243 - Brain dopaminergic neurons: In vivo electrochemical information concerning storage, metabolism and release processes☆

The potential usefulness of modern voltammetric techniques for investigations of brain neuronal functioning has gained increased interest in recent years. The advantages of the approach are basic: it enables one to measure concentrations of species directly, and it does this simply., quickly and in the presence of numerous other components in a complex system, negating the need to first isolate the molecular species of interest. Compensation for interfering species can be made in most cases. Voltammetric microelectrodes have the additional advantage of making measurements possible for the first time in vivo under conditions that approach normalcy, and in addition, of being instantaneous, localized, non-destructive, and selective towards the fluid phase. In this paper, cyclic voltammetry and differential pulse voltammetry at graphite paste electrodes are evaluated for their ability to detect the chemical neurotransmitter, dopamine, and its major metabolites, homovanillic acid and 3-methoxytyramine, in mammalian brain. Results are presented demonstrating the applicability of the methods developed to studies of transmitter storage, metabolism and release processes following selective pharmacological manipulations of the nigrostriatal dopaminergic projection. Certain problems inherent with such measurements in brain tissues are discussed.

L.e.e.d. and electrochemistry of iodine on Pt(100) and Pt(111) single-crystal surfaces

Abstract Pt(100) and (111) single-crystal surfaces containing adsorbed iodine were characterized by a combination of electron techniques in ultra-high vacuum (UHV) and electrochemical methods in aqueous solution. The low-energy electron diffraction (l.e.e.d.) patterns varied with coverage, indicating long range order of the overlayer and occasional registry with the substrate. The observed structures were hexagonal, exhibiting distortion in some cases. Quantitative Auger electron spectroscopy (a.e.s.) corroborated the iodine coverages inferred from l.e.e.d. Thermal desorption mass spectrometry (t.d.m.s.) revealed maxima in the iodine desorption rate from the (100) surface near 600 and 900 K, corresponding to specific surface structures, with a broad continuum in between; only a continuum was produced by Pt(111) surfaces. Atomic iodine is the predominant thermal desorption product. Electrochemical current-potential curves obtained by linear scan voltammetry in aqueous acid of ordered layers produced in UHV differ markedly for different faces and from those for adsorbed layers produced on the same substrates from aqueous solutions. Integration of voltammetric curves (coulometry) verified, for iodine overlayers, the effectiveness of a.e.s. for quantitative analysis of an adsorbed monolayer.

Orientational transitions of aromatic molecules adsorbed on platinum electrodes

Abstract A method to determine the orientation of molecules adsorbed at solid-liquid interfaces, based on thin-layer electrochemical techniques, is described. This method has been applied to determine orientational changes of molecules adsorbed from solution onto smooth platinum electrodes as a function of adsorbate concentration. Twenty-six diphenols and quinones, representing a variety of structures and chemical properties, were studied. In general, these compounds are adsorbed with the diphenol or quinonoid ring parallel to the surface at low concentrations and re-orient irreversibly to non-random edgewise-orientations as the concentration is increased. A possible explanation for these edgewise-orientations is discussed.

Superlattices formed by electrodeposition of silver on iodine-pretreated Pt(111); Studies by leed, auger spectroscopy and electrochemistry

Reported are studies by LEED and Auger spectroscopy of silver layers electrodeposited on well-characterized Pt(111) surfaces from aqueous solution. Prior to electrodeposition. the Pt(111) surface was treated with I2 vapor to form the Pt(111) (7 × 7)R19.1°-I superlattice which protected the Pt and Ag surfaces from attack by the electrolyte and residual gases. Electrodeposition of silver occurred in four distinct ranges of electrode potential. Ordered layers having (3 × 3) and (18 × 18) (coincidence lattice) LEED patterns were formed at all coverages from the onset of deposition to the highest coverages studied, about twenty equivalent atomic layers. Formation of ordered Ag layers has therefore been demonstrated, at least for deposits of limited thickness. Auger spectra revealed that for deposits of a few atomic layers. The iodine layer remained attached to the surface during multiple cycles of electrodeposition and dissolution of silver from iodine-free solution. Each peak of the voltammetric current-potential scan produced a change in the LEED pattern.

Electrodeposition on a well-defined surface: Silver on Pt(111)(√7×√7)R19.1°−I

Abstract An exploration is reported of the structures formed by electrodeposition of silver on well-defined Pt(111) surfaces in various amounts up to a few monolayers. Prior to electrodeposition, the Pt(111) surface was treated with I2 vapor to form a Pt(111)(√7×√7)R19.1°−I superlattice which effectively protected the Pt and Ag surfaces from attack by the aqueous HClO4 electrolyte and residual gases. Silver electrodeposited in three widely separated underpotential deposition stages, forming distinct lattice structures having (3×3) or (√3×√3)R30° LEED patterns at all coverages studied. Formation of ordered Ag layers has therefore been demonstrated. Measurements of Auger electron spectroscopic current for Pt, Ag and I revealed that the silver was located underneath the iodine atomic layer, which remained attached during multiple cycles of electrodeposition and dissolution of silver from iodine-free solutions.

Preparation of well-defined surfaces at atmospheric pressure: Studies of structural transformations of I, Ag-adlattices on Pt(111) by LEED and electrochemistry

Pt(111) surfaces disordered by ion-bombardment or electrochemical oxidation were converted to well-defined, ordered states by annealing in iodine vapor at atmospheric pressure. A structure not obtainable in vacuum was formed, Pt(111)(33 × 93)R30°-I, containing 0.62 I atoms per surfa ce Pt atom in a slightly distorted hexagonal array. The I-I interatomic distances in this structure, 0.33 and 0.36 nm, were less than the Van der Waals distance, 0.43 nm. Gentle heating of this structure under pure Ar yielded I2 molecules, I atoms and a series of structures: Pt(111)(33 × 9 3)R30°-I(3 × 3)R30°-IPt(111) (clean surface). The Pt(111)(7 × 7 )R19.1°-I adlattice proved to be identifiable from its distinctive electrochemical behavior in electrodeposition of Ag from aqueous solutions of AgClO4, which consists of three prominent structural transitions. Kinematic calculations of the directions and qualitative intensities of the LEED beams at selected kinetic energies has led to proposed structures consisting of Ag atoms close-packed in registry with the three-fold sites of Pt but with I atoms substituted for Ag atoms at the (3 × 3)R30° positions. Phase boundaries caused by reversals of the two packing sites of the 3 unit mesh at intervals 17 Pt unit vectors divide the surface into hexagonal antiphase domains.

Electrochemical oxidation of aromatic compounds adsorbed on platinum electrodes: The influence of molecular orientation*

Abstract Previous work has demonstrated that aromatic molecules adsorb on platinum electrodes in specific molecular orientations which change as the solution concentration is increased. The present article describes electrochemical oxidation of these adsorbed molecules as a function of orientation. The number of electrons, n ox , per molecular oxidized in aqueous 1 M HClO 4 was determined by thin-layer electrochemical methods. Twenty-nine compounds, representing a variety of structures and chemical properties, were studied: benzene; simple diphenols; alkyldiphenols; polyhydroxybenzenes; polyhydroxybiphenyls; tetrafluorohydroquinone; N -heteroaromatics; diphenols having surface-active side-chains; polycyclic phenols and quinones; hydroquinone mercaptans; and hydrogen sulfide. The magnitude of n ox is strongly dependent on initial molecular orientation, being smaller for edgewise than for flat orientations. These changes in n ox imply variations in oxidation product distribution with orientation, although direct identification of products was not made.

Adlattice structure and hydrophobicity of Pt (111) in aqueous potassium iodide solutions: Influence of pH and electrode potential

Abstract Measurements by means of Auger spectroscopy and LEED of the composition and structure of Pt (111) surfaces after immersion into aqueous iodide solutions are reported as a function of pH and electrode potential. Voltammetric current for reductive desorption of halogen was observed near −0.3 V (vs. Ag/AgCl reference). Comparison of the coulometric charge for halogen reductive desorption with the original packing density of iodine determined by Auger spectroscopy revealed that a one-electron reduction of the halogen had occurred: I(adsorbed) + e− → I−. The packing density of I atoms was a function of electrode potential, declining at the negative extreme of potential due to the reductive elimination process and also declining at the positive extreme of potential due to oxidation of the adsorbed halogen atoms and of the Pt surface. On the basis of LEED data, the halogen layer was found to be ordered in registry with the Pt (111) surface. The halogen adlattice structure was potential dependent: a (3 × 3) adlattice (ΘI = 4/9) at relatively positive potentials; a adlattice (Θ1 = 3/7) at potentials in mid-range; a adlattice (ΘI = 1/3) at relatively negative potentials; and a virtually halogen-free surface at potentials approaching the negative limit. The pH dependence of iodine adsorption was relatively slight, evidently because strong adsorption of halogen suppressed adsorption of OH and related species. The Pt (111)(3 × 3)-I and Pt (111) surfaces were remarkably hydrophobic, while the Pt surface was distinctly hydrophilic. Ionic species were not retained to a measurable degree at the hydrophobic surface, although normal retention of solution films and ions occurred at the surface in its hydrophilic states.

Superlattices formed by interaction of hydrogen iodide with Pt(111) and Pt(100) studied by LEED, Auger and thermal desorption mass spectroscopy

Abstract HI reacts dissociatively with Pt (100) and (111) surfaces to form gaseous H 2 and I-atom adsorbed layer structures identified as Pt (100)[ c (2 × 4)]- I (θ = 0.25 I-atom per surface Pt-atom), rings (0.34 ≤ θ Pt (100)[ c (2√2 × N √2)] R 45° − I (0.43 ≤ θ Pt (100) [ c (2√2 × √2)] R 45° − I ( θ = 0.50), Pt (111)(√3 × √3) R 30° − I ( θ = 0.33) and Pt (111)(√7 × √7) R 19.1° − I ( θ = 0.43). Each structure consists of an hexagonal array of I atoms, with slight distortion in some cases for best fit with the substrate. Quantitative Auger electron spectroscopy confirms coverages inferred from LEED. Thermal desorption mass spectroscopy reveals prominent rate maxima at 738 and 985 K, associated with desorption from the Pt (100) [ c (2√2 × √2)] R 45° − I and Pt (100)[ c (2 × 4)] − I structures, respectively, while the Pt(111) structures yield a small peak and a broad continuum. Atomic iodine is the predominant thermal desorption product for θ ≤ 0.50.