Ion Transport Through Synthetic Nanopores Deposited in Porous Manganese Oxide Wires

Abstract

Synthetic nanopores emerged as an important tool to understand ionic and molecular transport at the nanoscale. Properties of ion current passing through nanopores can reveal geometric as well as electrochemical characteristics of the structures. Ionic selectivity, for example, is indicative of the presence of surface charges, while ion current rectification indicates broken electrochemical symmetry in the form of patterned surface charge or geometry. In this study, we utilized synthetic nanopores to perform conductivity experiments on manganese oxide, a porous material whose electrical state can be modified. The measured ion current carried information on the effective size of the voids as well as the polarity of the surface charges. Membranes containing many pores, as well as single pores, were coated with a gold layer via sputter deposition, and then electrochemically deposited with manganese oxide wires. The gold layer extended inside the pores, in direct contact with the MnO2 wires, which permitted electrochemical modification of the wires. Measurements of ionic current through the wires were performed immediately after deposition, after an initial reduction with lithium, and after discharging the wires. These measurements revealed that each electrical state demonstrated different conductivites and provide strong evidence that the material has been successfully altered inside the nanopores. Experiments performed at a range of electrolyte concentration indicate the voids’ diameter of the porous MnO2 is dependent on the oxidation state of the material.