Structure and electrochemistry of adsorbates on rhodium(100) studied using scanning tunneling microscopy
Abstract
In order to gain an understanding of electrochemical processes at noble metal electrode surfaces, scanning tunneling microscopy and electrochemical methods were applied to the examination of a variety of adsorbate systems on a Rh(100) substrate. These systems included oxygen, iodine, metal halide clusters and thin films, DNA, and underpotentially deposited silver. A Rh(100)c(2 $\times$ 8)-O, $\theta\sb{O}$ = $7\over8$ adlattice was prepared using an atmospheric pressure dosing procedure. The STM images reveal a coincidence lattice structure in which the oxygen atoms are not bound exclusively to high symmetry sites but form a pseudo hexagonal lattice which undulates over the square Rh(100) substrate lattice. A sharp cathodic peak at -0.065 V was observed in the initial cyclic voltammetric scan of the oxygen dosed surface. Subsequent scans were similar to those obtained from a well-defined Rh(100) surface. A Rh(100)($\sqrt{2} \times \sqrt{2})$-I, $\theta\sb{I} = 1\over 2$ adlattice was prepared by cooling the annealed crystal in an I$\sb2$,N$\sb2$ gas stream. It was observed that additional iodine could be adsorbed over the initial ($\sqrt{2} \times \sqrt{2}$) iodine adlattice in the form of a (2$\sqrt{2} \times 9\sqrt{2}$) hexagonal overlayer. This affinity for additional iodine adlayers extends to metal halides as well. BiI$\sb3$ clusters, PbI$\sb2$ adlayers, and ordered HgI$\sb2$ monomers, to name a few, can be adsorbed on an iodine dosed surface and imaged using scanning tunneling microscopy. It was discovered that oxygen dosed Rh(100) is an excellent substrate for STM imaging of polydeosyadenylic acid. Additional stability of the molecule on the surface can be achieved through hydrogen bonding between the nucleotide and the adsorbed oxygen on the rhodium surface. Underpotential deposition of silver on the Rh(100)-I surface was investigated using ex situ STM. Three different structures were imaged upon emmersion of the crystal at the top of the three cathodic peaks present in the cyclic voltammogram for the UPD process.
Degree
Ph.D.
Advisors
Schardt, Purdue University.
Subject Area
Analytical chemistry|Chemistry
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