G. M. Bommarito

The Study of Solid/Liquid Interfaces with X-ray Standing Waves

The principles and applications of the x-ray standing wave technique are described. Emphasis is placed on its utility for the study of structure, composition, and distribution of interfacial species especially at solid/liquid interfaces.

Potential-dependent structural changes of underpotentially deposited copper on an iodine-treated platinum surface determined in situ by surface EXAFS and its polarization dependence

Abstract An in situ structural investigation of the underpotential deposition (UPD) of copper on an iodine-covered platinum surface (Pt/C layered synthetic microstructure with Pt as the outermost layer) has been carried out using surface extended X-ray absorption fine structure (SEXAFS) and its polarization dependence. The effects of rinsing the electrode with pure supporting electrolyte have also been investigated. At an applied potential of +0.20 V (corresponding to half a monolayer of electro-deposited copper) there are two in-plane CuCu distances: 2.56 ± 0.05 A and 4.21 ± 0.05 A. This suggests the coexistence of a close-packed phase with a more open phase. It is likely that the open phase is stabilized by repulsive interactions between partially charged copper atoms, as well as by the presence of the strongly adsorbed iodine layer. At +0.10 V (corresponding to a full monolayer of electrodeposited copper) there is only one CuCu distance of 2.56 ± 0.05 A. Combining in-plane (parallel to the electrode surface) and out-of-plane (perpendicular to the electrode surface) SEXAFS measurements, we found that at full monolayer coverage, the copper UPD layer is incommensurate with respect to the platinum surface. The iodine ad-layer rides the eletrodeposited copper forming a 3 × 3 unit cell. This interfacial structure is not altered upon rinsing with pure supporting electrolyte.

Structural studies of electrochemical interfaces with x rays

We have employed x‐ray absorption spectroscopy, including surface EXAFS (extended x‐ray absorption fine structure) and x‐ray standing waves (XSW) in the in‐situ structural study of electrochemical interfaces. The SEXAFS technique was used to study the structure of half a monolayer of copper underpotentially deposited on a Pt(111) electrode. From analysis of the data a Cu–Cu distance of 2.85 A and a coordination number of six were determined, strongly suggesting that the deposit is largely composed of clustered sites. The electrosorption isotherm of iodide on a Pt(111) electrode surface was measured in situ using x‐ray absorption spectroscopy, and it exhibited two plateaus which we ascribe to changes in packing density and surface structure. Using x‐ray standing waves, we have studied the potential dependence of the adsorption of iodide onto a Pt/C LSM and have interpreted the results in terms of distributional changes in the vicinity of the electrode surface.

X-ray Standing Wave Studies of Underpotentially Deposited Metal Monolayers

The x-ray standing wave technique has been employed in the study of the structure of underpotentially deposited (UPD) copper on an iodine covered platinum surface and of copper on a Au(100) single crystal electrode. For Cu UPD on Pt, surface coverage isotherms derived from both electrochemical and x-ray measurements are compared. The growth mode of the copper ad-layer appears to be strongly influenced by the electrode’s surface morphology. For Cu UPD on Au(100) the coherence of the adlayer is strongly dependent on the mode of deposition.

A Structural Investigation of an Ultra-Thin Langmuir-Blodgett Film by an X-Ray Standing Wave Excited in a LSM Substrate Under the Bragg Diffraction Condition

An organic multilayer on LSM substrate having the same period was investigated by XSW formed in LSM measuring the fluorescence from heavy atoms of Langmuir-Blodgett(L-B) film. The inclination of organic chains was determined and the interface between the film and LSM was shown to contain additional layer with thickness different for various parts of the sample.

In-situ structural studies of the underpotential deposition of copper onto an iodine-covered platinum surface using x-ray standing waves

We present initial results of an in situ structural investigation of the underpotential deposition of copper on an iodine covered platinum/carbon layered synthetic microstructure, using x-ray standing waves generated by specular (total external) reflection and Bragg diffraction. We also compare the result of surface coverage isotherms derived from both electrochemical and x-ray measurements.