Solving the "MoS2 Nanotubes" Synthetic Enigma and Elucidating the Route for Their Catalyst-Free and Scalable Production

This study solves a more than two-decades-long "MoS₂ Nanotubes" synthetic enigma: the futile attempts to synthesize inorganic nanotubes (INTs) of MoS₂ via vapor-gas-solid (VGS) reaction. Among them was replication of the recently reported pure-phase synthesis of the analogous INT-WS₂. During these years, successful syntheses of spherical nanoparticles of WS₂ and MoS₂ were demonstrated as well. All these nanostructures were obtained by VGS reaction of corresponding oxides with H₂/H₂S gases, at elevated temperatures (>800 °C), in a fluidized bed reactor (FBR) and a one-pot process. This success and apparent similarity between the two compounds "hid" from us the option of looking for the INT-MoS₂ reaction parameters in entirely different regimes. The main challenge in the synthesis of INT-MoS₂ via VGS was the instability of the in situ prepared suboxide nanowhiskers against over-reduction and recrystallization at high temperatures. The elucidated growth mechanism dictates separation of the reaction into five steps, as properties of the intermediate products are not consistent with a single process and require individual conditions for each step. A horizontal reactor with a porous-quartz reaction cell, which creates proper quasi-static (contrary to the FBR) conditions for the reaction involving sublimation, was imperative for the effective nanofabrication of INT-MoS₂. These findings render a reproducible synthetic route for the production of highly crystalline pure-phase MoS₂ nanotubes via a multistep VGS process, without the assistance of a catalyst and in a scalable fashion. Being a semiconductor, flexible, and strong, INT-MoS₂ offers a platform for much research and numerous potential applications, particularly in the field of optoelectronics and reinforcement of polymer composites.

Nanocomposites based on tubular and onion nanostructures of molybdenum and tungsten disulfides: inorganic design, functional properties and applications

The review concerns the development and the state-of-the-art in studies on the surface modification methods aimed at fabricating promising nanocomposites based on multilayer inorganic tubular and onion (fullerene-like) MoS2 and WS2 nanostructures. The synthetic details and structural features of these materials are considered. Considerable attention is paid to targeted functionalization of molybdenum and tungsten disulfide nanostructures and to fundamental principles that underlie their ability to chemical interactions. The functional properties and applications of the obtained materials are described.

The bibliography includes 183 references.

High Pressure Vibrational Properties of WS2 Nanotubes

We bring together synchrotron-based infrared and Raman spectroscopies, diamond anvil cell techniques, and an analysis of frequency shifts and lattice dynamics to unveil the vibrational properties of multiwall WS2 nanotubes under compression. While most of the vibrational modes display similar hardening trends, the Raman-active A1g breathing mode is almost twice as responsive, suggesting that the nanotube breakdown pathway under strain proceeds through this displacement. At the same time, the previously unexplored high pressure infrared response provides unexpected insight into the electronic properties of the multiwall WS2 tubes. The development of the localized absorption is fit to a percolation model, indicating that the nanotubes display a modest macroscopic conductivity due to hopping from tube to tube.

Enhanced field emission of WS₂ nanotubes

Results on electron field emission from free standing tungsten disulfide (WS2) nanotubes (NTs) are presented. Experiments show that the NTs protruding on top of microstructures are efficient cold emitters with turn-on fields as low as 1 V/μm and field enhancement of few thousands. Furthermore, the emission current shows remarkable stability over more than eighteen hours of continuous operation. Such performance and long-term stability of the WS2 cathodes is comparable to that reported for optimized carbon nanotube (CNTs) based emitters. Besides this, it is found that the WS2 cathodes prepared are less sensitive than CNTs in chemical reactive ambients. The high field enhancement and superior reliability achieved indicates a potential for vacuum nanoelectronics and flat panel display applications.

Structure analysis on the nanoscale: closed WS2 nanoboxes through a cascade of topo- and epitactic processes

Closed WS₂ nanoboxes were formed by topotactic sulfidization of a WO₃/WO₃·⅓H₂O intergrowth phase. The box-like morphology can be traced back to a topotactic dehydration reaction of the precursor followed by an epitactic induction of intermediate hexagonal WO₃.