AVS2001 Session EC-MoA: Electrochemical Control of Surface Structure: Growth and Dissolution

Monday, October 29, 2001 2:00 PM in Room 111

Monday Afternoon

Start	Invited?	Item
2:00 PM	Invited	EC-MoA-1 Local Dynamics of Electrochemical Dissolution and Growth Processes O. Magnussen (Universität Ulm, Germany) The active dissolution of a bare metal surface is of central importance for technological processes, such as aqueous corrosion, etching, and electrorefinement. Even on clean, single crystalline electrodes this electrochemical reaction proceeds at a highly non-uniform rate and is strongly localized at a small number of atomic-scale defects ("active sites") on the metal surface, which are commonly identified with kinks in atomic steps on the crystal surface. With the help of in situ scanning tunneling microscopy (STM) a detailed picture of the underlying microscopic mechanisms and dynamics is now emerging. Here, results on the dynamics of dissolution and growth at the solid-liquid interface by novel, STM-based methods with high temporal and spatial resolution are presented. Using high-speed Video-STM with image acquisition rates of up to 25 images per second the underlying atomic-scale processes, such as the nucleation and propagation of kinks along the steps of the crystal surface, can be directly studied, as illustrated for Cu(100) in HCl solution. In particular, it will be shown how the ordered Cl adlayer on this electrode surface determines the structure of active kinks as well as local reaction rates at these sites. Quantitative measurements of the kink propagation rates, i.e., the local dissolution/redeposition rates, reveal high local rates and pronounced local dissolution/redeposition fluctuations at the individual kinks. Even at the onset of Cu dissolution the average kink propagation rates and the average reaction rates at kink sites are in the range of 10³ and 10⁵ atoms s^-1, respectively. In addition, these studies give insights into the mechanisms of kink formation during metal dissolution/deposition, which can explain the characteristic surface morphology in this system. Together, these data give a coherent picture of the atomic-scale processes involved in the electrochemical dissolution and growth of Cu(100) in HCl solution.
2:40 PM		EC-MoA-3 In-situ STM Study of the Formation of Metal Nanostructures of Co on Ag(111) D. Marcu, N. Hall, S. Morin (York University, Canada) Recently, there has been a growing interest in the study of the formation of thin metal films and multilayer films of Ni, Co and their alloys due to their interesting magnetic properties. To optimise these materials, we must understand the nucleation and growth processes that are involved in the formation of individual layers. This includes monitoring surface alloying and intermixing processes taking place at the early stages of growth as well as the effect of varying the electrode potential on the formation of these metal films. During this presentation, the electrodeposition of Co and Ni¹] on the Ag(111) surface will be compared in terms of their nucleation and growth behaviour in similar electrolytes. Similar to Ni, Co electrodeposition proceeds via preferred nucleation and growth at the substrate step edges at high overpotential. At low overpotential values, the electrodeposition is very slow, with only a few islands nucleating at the step edges. Independently of island nucleation and growth, an adsorbed layer forms on the Ag terraces. This layer appears to be further reduced with time; possible explanations for the origin of this layer will be given. At high overpotential, there is strong evidence of Ag atom mobility, and the resulting substrate changes are only partially removed during Co dissolution. These results will be discussed in the view of possible surface alloying and intermixing between the Co and Ag substrate. These results contrast with previous studies concerning the electrodeposition of Co on Au(111),² in which no 2D island growth was reported. ¹S. Morin, A. Lachenwitzer, O.M. Magnussen, R.J. Behm, Phys. Rev. Lett., 83 (1999) 5066. ²L. Cagnon, A. Gundel, T. Devolder, A. Morrone, C. Chappert, J.E. Schmidt, P. Allongue, Appl. Surf. Sci., 164 (2000) 22.
3:00 PM		EC-MoA-4 Growth and Dissolution of Surface Structures by Electrochemical Control of Molecular Self-assembly Y. He, T. Ye, E. Borguet (University of Pittsburgh) Electrochemical control of molecular self-assembly offers a number of interesting perspectives ranging from molecular electronics to nanostructure formation. Self-assembled alkane monolayers have been observed on both reconstructed and unreconstructed Au(111) surfaces under electrochemical control with STM (scanning tunneling microscopy). The hexadecane molecules appeared as 2.2nm long rods, arranged in parallel rows in high resolution STM images, suggesting an extended molecular conformation. In situ STM enabled the influence of molecular overlayers on surface processes to be monitored in real time following potential driven perturbations. Our results indicate that self-assembled alkane monolayers modify the growth and dissolution of nanoscale islands and can serve as templates for selective deposition of other nanoscale structures.
3:20 PM		EC-MoA-5 Electrochemical Epitaxial Growth of Palladium Thin Layers on Au(111) and Au(100) Electrodes and Their Electrocatalytic Properties K. Uosaki, S. Ye, H. Naohara, T. Kondo, K. Tamura (Hokkaido University, Japan); M. Takahasi, J. Mizuki (Japan Atomic Energy Research Institute, Japan) It is well known that the physical and chemical properties of a metal ultrathin layer on a foreign substrate are different from those of the bulk metal.In the present study, the electrochemical epitaxial growth processes of Pd on Au(111) and Au(100) electrodes were investigated by using in situ scanning tunneling microscopy (STM)^1,2 and surface X-ray scattering (SXS).³ The effect of the thickness on the electrocatalytic activity for the anodic oxidation of formaldehyde at the Pd thin layers on the Au(111) and Au(100) substrates was also investigated.⁴ Electrochemical deposition of Pd was carried out in HClO₄ solution containing PdCl₄^2- complex. Electrochemical oxidation of formaldehyde on the deposited Pd layers was studied in HClO₄ solution containing formaldehyde. In situ STM measurements showed that palladium was grown epitaxially on both the Au(111)¹ and Au(100) substrates.² SXS measurements proved that the lattice parameters of the first Pd layer on Au(111) and Au(100) substrates were different from those of the bulk Pd crystal but same as those of the gold substrates.³ The electrocatalytic activity for oxidation of formaldehyde depends strongly on the structure and thickness of Pd thin layers. The peak current for formaldehyde oxidation at the Pd/Au(100) surfaces was much higher than that at the Pd/Au(111) surfaces. The peak potentials of anodic peaks corresponding to formaldehyde oxidation were shifted to negative direction as the thickness increased, reflecting the thickness dependent potential shift for oxide formation/reduction on the ultra thin layers of Pd. ¹H. Naohara, S. Ye, K. Uosaki, J. Phys. Chem. B, 102, 4366 (1998). ²H. Naohara, S. Ye, K. Uosaki, J. Electroanal. Chem., 473, 2 (1999). ³M. Takahashi, Y. Hayashi, J. Mizuki, K. Tamura, T. Kondo, H. Naohara, K. Uosaki, Surf. Sci., Surf. Sci., 461, 213 (2000). ⁴H. Naohara, S. Ye, K. Uosaki, J. Electroanal. Chem., 500, 435 (2000).
3:40 PM		EC-MoA-6 Electrochemical Deposition of Molybdenum Nanowires for Use as Sensors M.P. Zach, K. Inazu, J.C. Hemminger, R.M. Penner (University of California, Irvine) Electrodeposition of molybdenum dioxide (using the step edges of highly oriented pyrolytic graphite as nucleation templates) is used to form precursor nanowires ranging in diameter from 10 nm to one micron with many exceeding one-half millimeter in length. Subsequent treatment with hydrogen gas above 500°C reduces the wires to conductive metallic molybdenum.¹ Currently, this is the only method that exists to create millions of ordered nanowires with such high aspect ratios. A polymer film is used to lift the wires off of the conductive substrate thus isolating the wires electronically. The discovery of a method to deposit nanowires is just the beginning. A sensor device can be made by embedding the reduced wires in a polymer, laying a shadow mask over this array, and sputter-coating gold over the unmasked area. In the end, only the nanowires provide a conductive path across the gap left by the mask. The electronic properties of single wires can also be studied using a gold coated probes. The probes can also manipulate individual wires showing flexibility and mechanical properties of the reduced molybdenum and allowing conductivity measurements of single nanowires. Both the sensors and the single wire probe methods for measuring conductivity show decrease of conductivity as a function of time. Understanding the changes in resistance is imperative if these wires or wires made from other materials are to be integrated in functional devices. TEM, EDAX and XPS measurements have allowed characterization of the re-oxidation process in air. Freshly reduced molybdenum wires, upon exposure to oxygen and/or moisture, start to oxidize yielding thinner molybdenum metal wires with a insulating sheath of molybdenum trioxide. This data is important in order to understand the conduction and analyte adsorption properties of wires used in sensor devices. ¹M.P. Zach and R.M. Penner, Science, December 15, 2000, 290 (5499) 2120.
4:00 PM		EC-MoA-7 First Observation of Anomalous Scaling in Electrodeposited Films J.J. Mallett, W. Schwarzacher (University of Bristol, UK); N.M. Hasan, S.G. dos Santos (LSI/EPUSP/Universidade de Sao Paulo, Brazil) The evolution of surface roughness is of importance in most areas of surface physics and can provide useful insights into the nature of surface growth processes. Models of growth often predict surfaces that are statistically self-affine over a range of length scales l << l_c. For such surfaces, the interface width, w, scales as l^H, for small l, where l is the length scale over which the roughness is measured. The local surface roughness in the simplest models is found to be independent of the growth time. Many examples of this so called "normal" scaling have been reported to date. Some models also predict the possibility of anomalous scaling, in which the local roughness depends not only on l, but also on the growth time. In this case, known as anomalous scaling, the interface width scales as l^H tsuperβloc, ( l << l_c ). We report the first electrochemical results consistent with anomalous scaling and investigate the experimental conditions required to produce normal and anomalous scaling surfaces. Galvanostatic electrochemical deposition of copper from additive free acid sulphate solutions was performed onto gold substrates to produce films of various thickness. The interface width, w, was determined, as a function of l, by ex-situ atomic force microscopy. The type of scaling was found to be dependent on the ratio of the deposition current to the diffusion-limited current. At high values of this ratio, anomalous scaling was observed, while normal scaling prevailed at low values. This dependence was found to be independent of the solution concentration and temperature over the ranges measured. These results suggest that bulk diffusion of material to the electrode determines the type of scaling exhibited by the deposited surface.