In-Situ X-Ray Diffraction of Electrode Surface Structure

Andrea E. Russell, Stephen W.T. Price and Stephen J. Thompson University of Southampton, Chemistry, UK Diamond Light Source Ltd, Harwell, UK 

Developments in Electrochemistry Science Inspired by Martin Fleischmann, First Edition. 

Edited by Derek Fletcher, Zhong-Qun Xian and David E. Williams. 

In that first study, Fleischmann et al.[ ] relied on a high-surface area (20 g ) electrode 

The first in-situ X-ray diffraction (XRD) investigations of phase transitions of adsorbed monolayers and multilayers [6] and reconstmction of a metal surface [7,8] were also reported by Fleischmann and Mao. The phase transitions were reported for the underpotential deposition (upd) and overpotential deposition (opd) of thallium onto a roughened silver electrode surfaces (similar to those used in surface-enhanced Raman spectroscopy (SERS) using the reflection mode of collection), and for upd of lead onto gold and silver 

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Seikamoto K., Hirayama M., Sonoyama N., Mori D., Yamada A., Tamura K., Mizuki J. i., Kanno R. Surface Structure of LiNio.8Coo202 a New Experimental Technique Using in Situ X-ray Diffraction and Two-Dimensional Epitaxial EUm Electrodes, Chem. Mater. 2009, 21, 2632-2640. 

The electrochemical methods provide only indirect information about the structure of the UPD ad-atom layers in an atomic scale. In order to obtain more direct information about the structure of the UPD ad-atom layers, many investigators have adopted the use of in situ techniques, in which the electrode surface is examined with surface science methods. The methods mainly used are in situ X-ray diffraction [7,15-17], in situ scarming tunneling microscopy (STM) [18-23], and in situ atomic force microscopy (AFM) [24]. These methods have provided detailed information on the atomic structure, the thermodynamic stability, and the dependence of the structure on the potential for several UPD systems. Tfowever, little attempt has been made (and this mainly concerns the reduction of oxygen) to correlate directly the structure of the ad-atom... 

Recent decades have witnessed spectacular developments in in-situ diffraction and spectroscopic methods in electrochemistry. The synchrotron-based X-ray diffraction technique unraveled the structure of the electrode surface and the structure of adsorbed layers with unprecedented precision. In-situ IR spectroscopy became a powerfiil tool to study the orientation and conformation of adsorbed ions and molecules, to identify products and intermediates of electrode processes, and to investigate the kinetics of fast electrode reactions. UV-visible reflectance spectroscopy and epifluorescence measurements have provided a mass of new molecular-level information about thin organic films at electrode surfaces. Finally, new non-hnear spectroscopies such as second harmonics generation, sum frequency generation, and surface-enhanced Raman spectroscopy introduced unique surface specificity to electrochemical studies. 

Methods employing X-rays and y-radiation are used less often in electrochemistry. The possibility of using X-ray diffraction for in situ study of the electrode surface was first demonstrated in 1980. This technique has long been used widely as a method for the structural analysis of crystalline substances. Diffraction patterns that are characteristic for the electrochemical interface can be obtained by using special electrochemical cells and elec-... 

X-ray diffraction (XRD) is a routine method for determining crystal lattice parameters and molecular structure. The application of XRD to modified electrodes has been limited, particularly for actual molecular structure determination. First, such experiments presuppose a single-crystal electrode substrate. Second, the small amount of sample present in a thin film on an electrode surface means that the scattered intensities will be restrictively low, at least for commonly available x-ray sources [67]. However, if one is fortunate enough to have access to a synchrotron, such experiments are quite feasible. For details, the reader is directed to an excellent review by Toney and Melroy [68]. On the other hand, powder diffraction experiments with Cu or Mo Ka anode sources are straightforward, and can yield lattice-constant data in situ. For example, Ikeshoji and Iwasaki measured lattice constants for Prussian blue films (discussed earlier) on gold electrode surfaces [69]. 

Figure 2.9a shows the lipid molecule DMPC. Two layers contacted via the hydrophobic tails lead to spontaneous formation of a double-layer biomimetic membrane that can be transferred to a single-crystal ultraplanar electrochemical Au(lll) surface. The hydrophilic head groups contact the electrode surface via an intermediate water film. Due to the structurally very well-defined assembly, not only AFM and in situ STM but also neutron reflectivity. X-ray diffraction, and infrared reflection absorption spectroscopy (IRRAS) have been employed to support the direct visual in situ STM. Electrochemically controlled structural changes, phase transitions, and the effects of the common membrane component cholesterol (Figure 2.9b) and peptide drugs have been investigated in this way. 

The crystallographic structure of interphases can be investigated with various methods. In situ, the application of X-ray diffraction D4SEX is possible. Because of the depth of penetration of X-ray beams both in the transmission and the external reflection arrangement, the sample has to be made very thin in order to minimize unwanted contributions from the bulk of the electrode. Crystalline products of corrosion processes [47,48], surface films [49], surface reconstruction [50] and catalyst systems [51] have been investigated. 

New Techniques Surface X-ray diffraction is now a well-established technique for probing the atomic structure at the electrochemical interface and, since the first in situ synchrotron X-ray study in 1988 [6], several groups have used the technique to probe a variety of electrochemical systems. Most analysis has followed the methodology outlined in Sects. 4.1.2.1.1 to 4.1.2.1.3, whereby structural information at a fixed electrode potential is obtained by detailed measurement of the scattering from surface structures and modeling of the CTR profiles. In this section, a couple of recent applications of X-ray scattering are described that are particularly useful in studies of the electrochemical interface. 

In this chapter, we have attempted to show that in situ structural analysis techniques and traditional electrochemical measurements can be combined to obtain detailed insight into electrochemical reactions on single-crystal electrode surfaces. Thus, it is possible to establish a link between macroscopic electrochemical phenomena and atomic-scale surface structures. X-ray diffraction is a relatively new technique that takes advantage of the high brightness of synchrotron radiation sources to... 

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