A novel facile synthesis and characterization of molybdenum nanowires
© Kovic et al.; licensee Springer. 2012
Received: 1 August 2012
Accepted: 4 October 2012
Published: 13 October 2012
We describe a straightforward technique to synthesize pure Mo nanowires (NWs) from Mo6S y I z (8,2 < y + z ≤ 10) NWs as precursor templates. The structural transformations occur when Mo6S y I z NWs are annealed in Ar/H2 mixture leading to the formation of pure Mo NWs with similar structures as initial morphologies. Detailed microscopic characterizations show that large diameters (>15 nm) Mo NWs are highly porous, while small diameters (<7 nm) are made of solid nanocrystalline grains. We find NW of diameter 4 nm can carry up to 30 μA current without suffering structural degradation. Moreover, NWs can be elastically deformed over several cycles without signs of plastic deformation.
Synthesis and characterization of nanostructured materials have been a major area of research activities in the last two decades. Nanowires (NWs), nanorods, and nanobelts constitute an important class of 1D nanostructures which provide models to study the relationship between electrical transport, optical, and other properties with dimensionality and size confinements. Investigation of these nanomaterials has attracted much attention due to their wide range of potential applications in areas such as nanoscale circuitry linkages, field electron emitters, nanosensors, and magnetic devices. An important and promising method for preparing 1D nanomaterials is by using existing 1D nanostructures as templates via reactions such as metal to oxide with O2, oxide to metal with H2, metal or oxide to sulfide with H2S, carbon nanotubes to carbide with a vapor of metal oxide or halide, and copper oxide to copper. In particular, this approach exhibits good advantages when some 1D nanostructures might be difficult or impossible to synthesize directly. Metallic molybdenum is widely used in alloy, electrode, metal particle-toughened ceramic matrix composites, and catalysts, etc.[5–8]. There are several ways in which Mo NWs have been grown successfully: e.g., large-area-aligned Mo NWs synthesized by high temperature chemical vapor deposition for application as electron emitter, millimeter-scale length Mo NWs fabricated by electrochemical step edge decoration[2, 10], and atomic-scale Mo NWs grown inside double-walled carbon nanotubes as templates. Joule heating of Mo6S3I6 nanowires also causes transformation into Mo NW via thermal decomposition. The obtained NWs had 2-3 orders of conductivity higher than the starting material. Nanosized molybdenum materials have received much research and industrial attention because of their unique physico-chemical properties compared with the properties of corresponding materials with larger grains including better catalytic activity and selectivity for hydrogenation. Moreover, oriented mesoporous MoO3 thin films have been tested for battery application due to ease with which lithium ions can be stored in their van der Waals gaps. Increasing porosity and surface area are greatly desirable to enhance the faradaic capacitance and therefore would increase charge storage capacity. Herein, we present a novel and efficient route to the synthesis of various morphologies of Mo NW using hydrogenation of bundles of Mo6S y I z NWs as a precursor material. The method is fairly facile, scalable, and offers precise control over morphology and orientation of the end-products. Moreover, hydrogenation of large bundles (of diameters >15 nm) results in highly porous Mo NWs. These NWs are a promising material in a wide range of different applications ranging from solid state ionics, catalysts, nanoelectronic interconnects, flat panel field-emission devices, as well as fillers in composite materials. Small diameter Mo NWs (<7 nm) are made of nanocrystalline grains and can be produced by selecting similar diameters of the parent precursor. Their current-voltage (I-V) characteristics display metallic behavior with currents up to approximately 30 μA, for 4 nm NW, before breakdown. Such a high current density makes these NW suitable as inner interconnects in nanoelectronics.
The precursor crystals and synthesized products have been studied by high-resolution 200 keV JEOL 2010F (JEOL Ltd., Tokyo, Japan) field-emission transmission electron microscopes (HRTEM), 80 KeV JEOL JEM-2200FS double Cs-corrector TEM and scanning electron microscope FE-SEM, Supra 35 VP. Samples were monitored by X-ray powder diffraction (XRD) using a diffractometer Bruker AXS D4 Endeavor (Bruker Corporation, Karlsruhe, Germany) with Cu-Kα1 radiation and Sol-X energy dispersive detector within the angular range 2 Θ from 6° to 73° with a step size of 0.04° and a collection time of 3 or 4 s. The samples were rotated during measurements by 6 rpm. Electrical properties of several individual Mo NWs were tested under Ar environment using a custom-built conducting atomic force microscopy (AFM) system. Lateral and vertical manipulations were achieved by constant force ranges between 0.2 to1 nN.
Results and discussion
In conclusion, an efficient, low-cost and scalable method has been developed to fabricate pure Mo nanowires. At first, a selected morphology of Mo6S y I z is grown directly from the elements then followed by hydrogenation at 730°C. We find that the overall morphology of the synthesized Mo nanowires shows a one-to-one correspondence with the initial parent materials. This has allowed us to synthesize a variety of Mo NWs of relatively uniform diameters and lengths, including ones with aligned nanowires on free-standing foils. Large diameter Mo nanowires (>20 nm) are highly porous and can be selectively produced by controlling the growth parameters and the stoichiometry of the starting material. Such nanostructures have a very large surface area and can be advantageous for use as a host material for Li ion batteries or as fillers in composites. Oriented Mo nanowires of various sizes have also been grown directly on substrates and can be integrated into different device architectures such as field-emission devices and other nanoelectronic applications. Interestingly, narrow-diameter Mo NWs (<7 nm) are flexible, highly conductive, and carry relatively large current without suffering structural degradations.
AK is PhD student in school of International Studies of Jozef Stefan Institute. AZ is a PhD student in Ljubljana University. AJ is a research scientist from Jozef Stefan Institute. AM is a senior research scientist from Jozef Stefan Institute. MG is a professor from Ljubljana University and AH is a senior research scientist from National Institute of Industrial Science and Technology (Japan) and currently a senior research scientist from National Institute of Chemistry.
The financial support from the Centre of Excellence Low Carbon Technologies (CO NOT) is fully acknowledged. AH would like to acknowledge nanomicroscopy center at Aalto University for the use of double Cs-corrector TEM (JEOL JEM-2200FS) and Dr. Hua Jiang for the help with the instrumentation.
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