Low Potential Asymmetrical Functionalization of Conductive Micro- and Nanowires By Closed Bipolar Electrochemistry

Abstract

Bipolar electrochemistry occurs when faradaic reactions are driven on electronic conductor not directly connected to an electrical power source, but in contact with a dielectric (typically a solution) which is polarized under the influence of an applied electric field. Bipolar electrochemical cells can be designed according to two configurations: open bipolar electrochemical cells (OBPECs) 1-3 and closed bipolar electrochemical cells (CBPECs) 4,5. In OBPEC, the bipolar electrode (BPE) is a conductor immersed in an electrolyte solution. When a voltage is applied between two driving electrodes (not in direct contact with the BPE), a potential difference between the two poles of the BPE and the electrolyte solution is generated by the polarization of the latter. The extent of polarization at the two ends of the BPE is directly proportional to the intensity of the electric field and inversely proportional to the critical dimension (e.g. length) of the BPE. As a consequence, for performing open bipolar electrochemical depositions on micro or nanosized substrates, the applied electric fields must be of the order of tens kV/m. 6-7 In CBPECs, the BPE can be a whole conductive object or a composite material in which conductive particles are embedded in an insulating matrix. The main advantage of closed bipolar electrochemistry is the possibility to perform bipolar electrochemistry experiments by applying low intensity electric fields, and to operate even in concentrated electrolyte solutions. Furthermore, the cell is composed of two separated compartments, so that it is possible to use two electrolyte solution with different chemical composition and operating in distinct experimental conditions (e.g. temperature, pH, viscosity etc.). Therefore the use of CBPECs allows one to couple electrochemical reactions and to apply experimental conditions inaccessible via OBPECs. We demonstrate here the possibility to apply closed bipolar electrochemistry for the asymmetrical deposition of metals and metal oxides on bipolar electrodes of decreasing dimensions, down to nanosize by applying a potential difference between the driving electrodes as low as 1 V. In particular, we focus on the asymmetrical deposition of semiconducting oxides (TiO2, Cu2O and Co2O3) and Pt metal at the two opposite ends of carbon-microwires and gold-nanowires (30 nm diameter). The optimization of the process is studied using a 4-electrode voltammetric cell.8 Scanning electron microscopy and energy dispersive X-ray spectroscopy confirm the achievement of the desired deposition. Electron backscatter diffraction is applied to perform the crystallographic analysis identifying cuprite in all the Cu2O deposits. The deposited materials are chosen on the basis of their photochemical, photoelectrochemical and electrocatalytic properties, moreover they require deposition conditions which are hardly applicable in OBPECs. The results achieved demonstrate that closed bipolar electrochemistry can be successfully applied to obtain the Janus-like nanosized objects containing two different hetero-junctions of the kind: semiconductor 1/metal nanowire/ semiconductor 2.