Solubility of Mo₆S_4.5I_4.5 Nanowires in Common Solvents:  A Sedimentation Study

Abstract

Due to their ease of fabrication and monodisperse, metallic nature, molybdenum-sulfur-iodine nanowires are an interesting alternative to carbon nanotubes for some applications. However very little is known about the solubility of these materials. In this work we have investigated the solubility of Mo(6)S(4.5)I(4.5) nanowire soot in a range of common solvents by performing sedimentation studies and microscopic and spectroscopic characterization. A sedimentation equation was derived showing that the concentration of any insoluble dispersed phase decreases exponentially with time. We find that in all solvents, Mo(6)S(4.5)I(4.5) nanowire soot contains three phases, two of which are insoluble with one stable phase. Microscopy and spectroscopy show that the first insoluble phase is associated mainly with spherical impurities and sediments rapidly out of solution resulting in purification. The second phase appears to consist of insoluble nanowire bundles and sediments more slowly, eventually leaving a stable dispersion of nanowire bundles. The stably dispersed bundles tend to be smaller than their insoluble counterparts. The best solvents studied were 2-propanol and dimethylformamide. Microscopy studies showed that, in the case of 2-propanol, sonication significantly reduced the bundle size relative to the unsonicated bulk. However, during sedimentation, large quantities of bundles were observed to reaggregate to form larger bundles which subsequently sedimented out of solution. In general, the sedimentation properties of the various phases did not vary significantly with concentration indicating that the insoluble nanowires are intrinsically insoluble. However, the diameter of the stably dispersed bundles decreased with concentration, until very small bundles consisting of only two or three nanowires were observed at concentrations below 0.003 mg/mL. In addition, stable composite dispersions were produced by mixing the nanowires with poly(vinylpyrrolidone) in 2-propanol opening the way for the formation of polymer/inorganic nanowire composites.

Liquid Phase Production of Graphene by Exfoliation of Graphite in Surfactant/Water Solutions

“…The measured absorbance fell steadily, indicating sedimentation of approximately two thirds of the material over a considerable period of time. A bi-exponential function could be fitted to the profile, indicating one stable and two sedimenting components 18 . The fit parameters indicate that 35wt% of the sample is stable over the timeframe of 35 days.…”

“…Of the rest, 19 wt% of the flakes fall out rapidly, with a time constant of 21.5 hrs while a further 46 wt% fall out over longer time scales (time constant ~208 hrs). As the time constant is related to the dimensions of the sedimenting object 18 , we can attribute the slowly and rapidly sediment objects to medium and large sized flakes respectively. We suggest that the large flakes are fragments of graphite that inadvertently remained in the dispersion after decantation and which we can identify with the type of flake observed in Figure 4E.…”

“…Absorption measurements were taken with a Varian Cary 6000i using quartz cuvettes. Sedimentation profiles were taken with a home-made apparatus using an array of synchronised pulsed lasers and photodiodes 18 . Annealing of some deposited films was carried out in a GERO Hochtemperaturöfen GmbH.…”

Processing and characterisation of Mo6S2I8nanowires

“…5 This was monitored using the linear transmitted intensity I through the dispersion, which varies with C(t) as given by the Lambert-Beer law:…”

“…The sediment and solute morphologies for all solvents are summarised in Table SI1.w The sedimentation time constant t can be shown to increase with aspect ratio of the relevant phase, 5 so from a purely physical standpoint, the quasispherical impurities and bundles with a low aspect ratio should sediment out first, followed by thick bundles, leaving only thin ones in solution.…”