Understanding and directing electrochemical reactions below the micrometer is a long-standing challenge in electrochemistry. In this respect, electrodeposition using scanning probe microscopy provides an attractive platform, not only for additive nanoscale manufacturing, but also for the investigation of local dynamics such as mass transport and reaction kinetics.
We report on the direct writing of copper nanostructures using an oscillating nanoelectrode operated with an Atomic Force Microscope (AFM) in aqueous solutions. The nanoelectrode is used to locally initiate electrochemistry, while the AFM allows for in-situ probing of the deposited material. We demonstrate the writing of arbitrary structures beyond cluster size with lateral dimensions in the order of the tip radius.
Strikingly, this local writing is only possible in highly dilute electrolytes (~ 1 µM CuSO4). Moreover, we find a profound effect of the dynamic operation of the nanoelectrode on the deposition kinetics.
We explain our results by considering the extended screening length and slow charge dynamics of the dilute electrolyte, which allows the nanoelectrode to operate inside, and disturb, the electrical double layer (EDL) at the solid-liquid interface. We visualize these (dis)charging dynamics and their effect on the reaction directly through the electrodeposited structures, and experimentally observe the Debye time of the electrolyte as a threshold value for growth. Furthermore, we show that this threshold time increases as a function of applied potential.
Our results lead us to propose a reaction mechanism based on cyclic charging and discharging of the EDL, which facilitates local electrodeposition through direct electrostatic interaction between ions in the liquid and the nanoelectrode.