Synthesis of molybdenum nanowires with millimeter-scale lengths using electrochemical step edge decoration

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 MoO₄^2- 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, t_dep, produced smooth MoO₂ nanowires with diameters proportional to t_dep^1/2. 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.