Tailor synthesis of 0D, 1D and 2D transition metal dichalcogenide nanostructures
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Abstract
The work presented in this doctoral thesis is a compilation of different approaches on the synthesis of transition metal dichalcogenides (TMDs) and the discussion on their probable formation mechanism. As the first step, a combination of metal organic chemical vapor deposition and chemical vapor transport approaches was taken to synthesize MoS2 nanotube bundles and iodine was used as a mineralizing agent. The reaction was carried out in a horizontal tube furnace. In the absence of iodine, no nanotubes were formed. Therefore, it is demonstrated that the iodine content of the precursors led to the enhanced mobility of the Mo and S constituents, the formation of point defects within the MoS2 layers and finally scrolling of MoS2 nanosheets. rnThis synthetic method was further extended to synthesize WS2 nested fullerenes and the formation of core-shell 2H-WS2@IF-WS2. Intermediate nanoparticles were studied using different electron microscopy techniques. The internal volume of the nested fullerenes was studied using a combined scanning electron microscopy/focused ion beam technique by cutting the cross sections of the core-shell nanoparticles. The lamellar reaction intermediates were found occluded in the fullerene particles. The role of the reaction and annealing temperature on the composition and morphology of the final product were also investigated. The stiffness of the WS2 shell was measured using intermittent contact-mode AFM.rnIn addition, a facile route for the synthesis of WS2 nanotubes starting from solvothermally derived tungsten oxide nanowires was demonstrated. Defect-rich multiwalled WS2 nanotubes were made by reductive sulfidization of W18O49 nanowires that were obtained solvothermally from WCl6 in different alcohols. W18O49 nanowires were also synthesized using a hot injection method, but these nanowires failed to form WS2 nanotubes. The defect-rich nanotubes were highly dispersible in organic solvents and were easily functionalized by Au, MnO and Pt@Fe3O4 Janus nanoparticles on the basis of Pearson’s HSAB principle which proved the direct transfer of defects from the precursor to the end product. rnIn order to investigate whether the preservation of the morphology applies for any other structures rather than 0D and 1D precursors, the oxide to sulfide conversion method was utilized to convert WO3 low aspect ratio nanorods to corresponding sulfides. In this case, nested tungsten sulfide geometrical nanoparticles with 90° apex described as “nanocoffins” were obtained and the automated diffraction tomography was used to investigate the effect of the oxide precursor crystal structure on the final morphology of the sulfide product. The box-like morphology was shown to originate through topotactical dehydration reaction of the precursor, i.e. a WO3•1/3H2O crust on WO3, followed by epitactic induction of intermediate hexagonal WO3 which serves as a template to maintain the particle shape in final product. In fact, a cascade of topotactic reaction leading to epitactic induction leads to the formation of closed rectangular boxes made from hexagonal layers.rnAs a step further, a lithiation/exfoliation approach was taken to synthesize a new class of TMDs in 2D form. Here, the restriction of convenient layered chalcogenide nanoparticles toward the intercalation was overwhelmed by selecting Nb1-xWxS2 coin-roll nanowires (CRNWs) as appropriate intercalation host. CRNWs were intercalated using n-BuLi in an inert atmosphere and the exfoliation of the Li-intercalated CRNWs using H2O led to the formation of graphene-type sheets of Nb1-xWxS2. It was demonstrated that the in situ functionalization of the graphene-type sheets using gold nanoparticles enhanced the stabilization of these sheets in the aquatic solution.