Double Layer of a Gold Electrode Probed by AFM Force Measurements

Abstract

Colloidal probe atomic force microscopy was used to determine the electric double layer interactions between a gold electrode and a spherical silica probe. The double layer properties of the gold/solution interface were varied through the pH and salt concentration of the electrolyte, as well as by externally applying an electric potential. The double layer potentials ψ(d) of the gold surface were obtained by fitting the force-distance curves according to the DLVO (Derjaguin-Landau-Verwey-Overbeek) theory, using earlier obtained values for the double layer potential of the silica probe as input parameter. It was found that the gold electrode combines the features of reversible and polarizable interfaces; i.e., its charge and potential are determined by both the solution pH and the external potential. The pH dependence is attributed to proton adsorption and desorption from oxidic groups on the gold surface. In the potential range studied, ψ(d) varies linearly with the applied potential; the variation in ψ(d) is roughly 10% of that in the applied potential. The potential of zero force (the external potential at which ψ(d) = 0) varies with pH. The various features of the gold/electrolyte interface are described well by an amphifunctional double layer model. The results of this study form the basis of the interpretation of adsorption studies on gold as a function of pH and externally applied potential.

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…Despite the qualitative agreement, there are differences between the results: The force observed between silica and gold changed almost linearly with the applied potential in a range from À0.6 to +0.4 V SCE (V SCE is the potential at the saturated calomel electrode) [338,460]. Since the force curves reflect the surface effective surface potential of the gold surface this indicates that the surface potential also changes over a range of 1 V. Raiteri et al observed a change of the force only in a narrow potential range of roughly 0.4 V. Above and below this voltage range the force saturated.…”

“…Dedeloudis et al probed the electric double layer on a copper electrode with a silica sphere [461]. The results obtained with silica microspheres on gold [338,460] and with microfabricated silicon nitride tips [457] qualitatively agree. In all cases the silica microsphere (or tip) is repelled at negative potentials of the gold.…”

“…This saturation behavior is also predicted by DLVO theory. The different potential range, where the force changed, can be caused by a large potential drop across an oxide layer or a Stern layer on the gold surface [460], which causes a large potential and might have been present in Refs. [338,460] but was less significant in Ref.…”

“…Campbell and Hillier determined the force between a silica sphere as probe and carbon or gold [338,459]. Barten et al concentrated on the gold surface probed by a silica sphere [460]. Dedeloudis et al probed the electric double layer on a copper electrode with a silica sphere [461].…”

The Intersection of Interfacial Forces and Electrochemical Reactions

“…DLVO theory was found to do an excellent job of predicting interaction forces when the mica−gold separation distances were greater than about one Debye length, 3 and several subsequent studies of interfacial forces under external potential control reached similar conclusions. 5,11,13,14 Further, the magnitudes of the double layer forces were found to saturate at electrochemical potentials that are large in magnitude: increasing the (negative) magnitude of the applied potential from −600 to −700 mV resulted in no measurable change in the magnitude of the measured double layer repulsion ( Figure 4b). 3 The presence of a force saturation regimewhere large increases in surface potentials lead to marginal increases in double layer forcesis also predicted by DLVO theory ( Figure 4a).…”