Sharply defined lateral composition gradients of copper on gold by spatiotemporal control of the in-plane electrochemical potential distribution

Abstract

Injecting an in-plane current by connecting the two ends of a thin Au film to different potentials relative to a common reference couple results in the formation of a linear potential gradient at the surface of the electrode. Coupling the surface potentials to a redox species in solution then causes spatially dependent redox chemistry, effectively mapping the electrochemical reaction onto the surface. Copper was selectively deposited to the surface of the Au film by selecting applied potentials that encompass E eq for the Cu 2+/Cu 0 reduction. Upon potential application copper deposits onto the area of the electrode for which V( x)< E eq, while the areas with local potentials positive of E eq contained no observable Cu. A transition region is established on the surface in an area corresponding to the anodic peak (strip-out) potential. Careful calibration of the surface electrochemical potential gradient indicates that the transition occurs at ca. 116 mV in agreement with an anodic peak potential centered at ca. 125 mV vs. Ag/AgCl from cyclic voltammetry. The width of the interfacial region analyzed by Auger electron spectroscopic line profiling and atomic force microscopy (AFM) is found to be dramatically sharper than expected. For a reversible two-electron process the development of surface composition based on a Nernstian local potential would yield a transition region ∼59/2 mV wide. Translated to the spatial domain the interfacial width should be ∼243 μm when a 123 mV/mm electrochemical potential gradient is imposed. Auger and AFM data confirm the interfacial width is strikingly smaller—by a factor of approximately 10—strongly suggesting that either the local electrochemical potential, V( x), is altered significantly upon Cu deposition, or V( x) alone is not responsible for the sharpness of the interface.