Synthesis of Molybdenum Nanowires with Millimeter-Scale Lengths Using Electrochemical Step Edge Decoration

Abstract

Molybdenum (Mo0) nanowires with diameters ranging from 13 nm to 1 μm and having lengths of up to 500 μm have been synthesized by a two-step process involving the electrodeposition of molybdenum dioxide (MoO2) on a highly oriented pyrolytic graphite (HOPG) surface followed by reduction of the MoO2 nanowires at 500 °C to form molybdenum metal. MoO2 nanowires were electrodeposited reductively from an alkaline MoO42- solution using a low overpotential of <|200 mV|. Under these conditions, the nucleation of MoO2 occurred selectively at step edges present on the HOPG surface, and a high linear density (>30 μm-1) of MoO2 nuclei was formed along each step edge on the graphite surface. With continued growth, MoO2 nuclei that were 10−15 nm in diameter coalesced with adjacent nuclei along the step edge to form continuous nanowires having approximately this same diameter. Longer deposition times, tdep, produced smooth MoO2 nanowires with diameters proportional to tdep1/2. These nanowires had a uniform diameter and a smooth surface, and the nanowire-to-nanowire variation in diameter was 10−30%. In contrast to nanowires of MoO2, which were brittle and subject to breakage, molybdenum metal (Mo0) nanowires were resilient and resistant to breakage. Arrays of Mo0 nanowires could be embedded in a polymer film and lifted off the graphite surface. In humid air (≈55% relative humidity) at room temperature, the resistivity of Mo0 nanowires increased by 10% h-1 as an insulating surface oxide of MoO3 formed.