Fundamental Properties of the Electrified Solid-Liquid Interface

We examine electrode surfaces in order to elucidate properties of the electrochemical double layer and focus on fundamental properties of the electrified solid-liquid interface.  In this set of projects, we use probe microscopic and vibrational spectroscopic methods to examine fundamental aspects of the electrified interface.   There are two general areas of activity.  First, we use the relatively new technique of infrared visible sum frequency generation (SFG) spectroscopy to examine the structure of water at the electrified solid-liquid interface.  For decades, it has been appreciated that the structure of water near the electrode surface might be different from that in bulk solution.  These differences are directly related to aspects of electrochemical activity.  However, until recently, it has been difficult to obtain direct information about the water structure at the electrode surface.  SFG is a vibrational spectroscopic technique sensitive only to material at interfaces.  We have successfully utilized this technique to examine the structure of water on a Ag surface, and showed that the presence of electrolyte (necessary for any electrochemistry) completely changes the structure of water above the interface.  In control studies performed without electrolyte, only weakly hydrogen-bonded water was present.  However, with electrolyte, the SFG reveals the presence of both strongly H-bonded water and water bound to the Ag electrode surface as shown in Figure 1.  The presence of these additional types of water has many consequences for understanding the electrified interface. Current efforts are directed toward understanding the effect of electrolyte charge, size, and shape on solvent structure above an electrode.

Figure 1: SFG spectra of OH stretches of water obtained from a Ag(100) surface in 0.1 M KF electrolyte at the applied potentials indicated.  The black lines are the experimental data points, the red lines are the fitting results, and the blue lines are the fit decomposition.

A second project in this area utilizes ‘atomic force spectroscopy’ performed by measuring potential dependent force curves using the atomic force microscope (AFM).  Interactions between an AFM tip and a surface depend intimately on the nature of the tip, the surface, and the intervening media. Electrochemical potential can be adjusted to change all three of these parts in a reproducible, controllable manner, so that we can probe features of the surface and solution above the surface.  We used potential dependent force spectroscopy to interrogate the Cu surface in basic solution (using an SiO­ modified tip) as shown in Figure 2 and have recently completed work examining the potential dependent desorption of thiols of different chain lengths from a Au tip and a Au surface.  We are now examining the surface chemistry of Fe and related materials.

Figure 2:  Potential dependent adhesion curves obtained from a Cu(111) surface in 0.1 M KOH.  Red line is the anodic scan while the blue line is the cathodic scan.  The transients correspond to exposure of different surface chemistries during the potential sweep.