The first inorganic nanopods and nanobuds

author: Marko Viršek, Jožef Stefan Institute
published: Jan. 18, 2008,   recorded: October 2007,   views: 382
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The discovery of WS2 and MoS2 nanotubes and inorganic fullerene-like nanoparticles (IF), shortly proceeded the discovery of carbon fullerenes (C60), nanotubes and onions. The WS2 and MoS2 nanomaterials have shown important applications as solid lubricants, electron devices, catalysts, super shock absorbers, etc. In tribology, on contrary to plate-like crystals, IF exhibit ultra-low friction and wear even in humidity. The mass production of non-agglomerated IF is an important challenge. We report on new production approaches of new forms of IF in macroscopic quantities. The synthesis is based on sulphurization of MoOx, WOx and MoxSyIz quasi one-dimensional precursors, which transform to MoS2 or WS2 based IF nanomaterials. The results of various characterisation methods reveal the possible mechanism of the formation of these new complex nanomaterials. W5O14 nanowires are synthesized by chemical transport reaction using NiI2 as a growth promoter. The light-blue crystals of metallic conductivity with specific resistivity of 27 cm have grown up typically to several millimetres in length and up to 200 nm in wideness. This rarely synthesized phase was reported as homogeneous phase only in 1978 by I.J. McColm et al., and in meantime declared as the compound, which may hardly exist. The W18O49 nanowires up to several millimetres in length are synthesized by a chemical transport reaction using iodine as a transport agent. The morphology of the wires can be controlled by the ratio between starting materials and by the growth conditions. By optimisation we can gain pure purple W18O49 phase with crystals several hundred nanometers wide or blue nanowire W18O49 phase with wideness bellow 200 nm. The first inorganic nanobuds – WS2 nanotubes decorated on outer surface with fullerene-like particles are synthesized by sulphurization of the W5O14 nanowires. The sulphurization takes place in a gas mixture of 1%H2, 1% H2S and 98 % Ar with a flow rate of 30 ml/min at 1050 K for 2 hours. The fullerene-like particles nucleate in surface corrugations of the nanowires and grow up by a diffusion process simultaneously with the transformation of nanowires to hollow multi-wall nanotubes. At slightly lower sulphurization temperature the material was transformed to WS2 nanotubes. The diameter of the tubes or nanobuds is smaller than the wideness of precursor nanowires revealing an exfoliation of the precursors. The sulphurized W18O49 nanowires transform to nanotubes decorated with flakes of WS2, but not forming the fullerenes-like particles. Nanowires based on Mo6S2I8 were also used as precursor crystals for sulphurization. We synthesized the Mo6S2I8 nanowires directly from elements at 1320 K. The prolonged reaction time (72 hours) resulted in several millimetre long needles having a diameter from several tens to a few hundred nanometers. The result of the sulphurization are the first MoS2 nanopods or “mama-tubes” - spherical MoS2 nanoparticles grown in the confined geometry of MoS2 nanotube reactors.

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