The examination of adsorbates on well-defined metal electrode surfaces using scanning tunneling microscopy

Carissima Marie Vitus, Purdue University

Abstract

Towards understanding fundamental electrocatalytic processes, scanning tunneling microscopy and electrochemical methods were used to examine various adsorbates on well-defined metal electrode surfaces. Towards elucidation of chemical processes under reaction conditions, in-situ scanning tunneling microscopy under direct potential control on various systems was also performed. Atomic images of Pt(100) c($\sqrt{2}\times5\sqrt{2})R$45$\sp\circ$-Iodine, $\theta\sb{I}$ = ${6\over 10}$, prepared by gas-phase dosing of iodine in a I$\sb2$/N$\sb2$ carrier gas stream, was observed in air. Images of iodine, adsorbed onto well-ordered Pt(100) from 1mM KI in 0.1M HClO$\sb4$, revealed two structures; a c($\sqrt{2}\times5\sqrt{2})R$45$\sp\circ$ adlattice and a c($\sqrt{2}\times 2\sqrt{2})R$45$\sp\circ$, $\theta\sb{I}={2\over4}$, structure at potentials negative of the 5$\sqrt{2}$ structure. Another adlattice was observed when iodine was adsorbed onto well-ordered Pt(100) prepared using a quenching method, namely, the ($\sqrt{2}\times\sqrt{2})R$45$\sp\circ$, $\theta\sb{I}={1\over2}$, structure. These structures were also observed in perchloric media. Silver underpotential deposition on Pt(111) ($\sqrt{7}\times\sqrt{7})R$19.1$\sp\circ$-Iodine proceeds through a series of narrow voltammetric peaks corresponding to structural transformations in the growing silver deposit. Atomic images of the silver electrodeposit in the initial peak revealed the formation of an asymmetric (3 x 3) (Ag,I) commensurate adlattice. This adlayer was similarly observed using in-situ STM. Images throughout the second peak showed the formation of ($\sqrt{3}\times\sqrt{3})R$30$\sp\circ$ (Ag,I) adlayer. Later stages of silver electrodeposition resulted in the formation of a silver (18 x 18) superlattice incommensurate with the substrate, similar to the observed structure imaged using in-situ STM at potentials near the end of the second peak. Implementation of an attachable electrochemical cell for our in-situ microscope allowed for direct atomic imaging of electrochemical processes under reaction conditions. Using this cell, the effects of electrode potential on saturation coverages of carbon monoxide on Pt(100) and Rh(100) surfaces were examined. Local (1 $\times$ 1) microdomains induced by CO adsorption on iodine-coated Pt(100) at negative potentials were observed. Sweeping the potential close to the potential for CO electrooxidation resulted in island coalescence. A Rh(100) c(2 $\times$ 4), $\theta\sb{CO}$ = $6\over 8$, unit cell was observed at initial negative adsorption potentials. This structure was continuously imaged throughout an electrode potential sweep.

Degree

Ph.D.

Advisors

Schardt, Purdue University.

Subject Area

Analytical chemistry

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