Surface Analysis for Electrochemistry: Ultrahigh Vacuum Techniques

The study of processes that transpire at heterogeneous interfaces is an exceedingly difficult proposition. No single experimental technique can ever hope to unravel all the nuances of heterogeneous reactions; hence, in surface science, the use of multiple complementary methods is not uncommon. Ultrahigh vacuum electrochemistry (UHV/EC) is a term ascribed to the approach that rests upon the integration of classical electrochemical methods with surface-sensitive analytical techniques; this strategy parallels that successfully implemented in the study of gas–solid heterogeneous catalysis. The unique surface sensitivity of the techniques adopted emanates from the use of particles (e.g. ions or electrons) that serve to interrogate the outermost layer(s) of the electrode. This surface sensitivity is tempered by the requirement that the analysis be performed in an environment (outside the electrochemical cell) that does not impede the mean-free paths of the probe particles. Since its inception in the early 1970s, more than a thousand UHV/EC-based studies have been published; most of the work involved polycrystalline materials and focused on the elemental composition at the electrode surface. While its importance in the study of polycrystalline surfaces cannot be trivialized, the greater value of UHV/EC appears to be in its ability to help resolve fundamental issues that intertwine interfacial structure and composition with electrochemical reactivity. It is in this context that the present review is written. A complete mechanism of an electrochemical reaction must incorporate all the physical and chemical interactions that arise between an electrified surface and its environment. The extent of such interactions depends upon several factors such as solvent, supporting electrolyte, electrode potential, reactant concentration, electrode material and surface crystallographic orientation. The traditional approach is based upon a thermodynamic treatment of the interface and its response to external perturbations. Interpretation of the results relies on phenomenological models of the interface. Although a thermodynamic treatment cannot be ignored, the need for an atomic-level view has long been realized. One approach1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 towards the establishment of an atomic-level description parallels that successfully implemented in the study of gas–solid heterogeneous catalysis; it rests upon the integration of classical electrochemical methods with surface-sensitive analytical techniques. The analytical methods that exhibit surface sensitivity are based upon the mass-selection and/or energy-discrimination of electrons, ions, atoms or molecules scattered from solid surfaces. These particles have shallow escape depths; hence the information they bear is characteristic of the near-surface layers. Their short mean-free paths, however, necessitate a high vacuum (<10−6 torr) environment. The application of such surface techniques to electrochemistry requires that the analysis be performed outside the electrochemical cell. The possibility of structural and compositional changes that accompany the removal of the electrode from solution is the major concern in UHV/EC studies. Although a myriad of surface analytical techniques is currently available, those actually employed in UHV/EC have been limited to low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), reflection high-energy electron diffraction (RHEED), work-function changes, and thermal desorption mass spectrometry (TDMS). While most vacuum-based analytical methods do not require single-crystal surfaces, the use of uniform (monocrystalline) surfaces is a necessary aspect for fundamental studies. The low-index crystallographic faces [(100), (110) and (111)] have been widely used because of their low free energies, high symmetries, and relative stabilities. In addition, it may be possible to reconstruct the overall behavior of polycrystalline electrodes from the individual properties of the low-index planes. 1, 2, 3, 4 A handful of procedures for the preparation and preservation of well-defined single-crystal surfaces have been described. 5, 6, 7, 8 The verification or identification of initial, intermediate, and final interfacial structures and compositions is an essential ingredient in electrochemical surface science.