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

Abstract

This study solves a more than two-decades-long "MoS 2 Nanotubes" synthetic enigma: the futile attempts to synthesize inorganic nanotubes (INTs) of MoS 2 via vapor−gas−solid (VGS) reaction. Among them was replication of the recently reported pure-phase synthesis of the analogous INT-WS 2 . During these years, successful syntheses of spherical nanoparticles of WS 2 and MoS 2 were demonstrated as well. All these nanostructures were obtained by VGS reaction of corresponding oxides with H 2 /H 2 S 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-MoS 2 reaction parameters in entirely different regimes. The main challenge in the synthesis of INT-MoS 2 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-MoS 2 . These findings render a reproducible synthetic route for the production of highly crystalline pure-phase MoS 2 nanotubes via a multistep VGS process, without the assistance of a catalyst and in a scalable fashion. Being a semiconductor, flexible, and strong, INT-MoS 2 offers a platform for much research and numerous potential applications, particularly in the field of optoelectronics and reinforcement of polymer composites.

WS2 Nanotubes: Electrical Conduction and Field Emission Under Electron Irradiation and Mechanical Stress

“…Furthermore, W:O atomic ratio of about 1.0:2.5, very close to WO 2.72 , that can be ascribed to suboxide residuals (intermediate precursor) rest in the core of anomaly wide nanotubes (slow diffusion reaction of gases into wide tubes may not complete oxygen‐to‐sulphur conversion), which is in compliance with the INTs’ growth mechanism. [ 35,53 ] The S 2p well‐resolved peaks, the W 4+ and S 2− species character, in agreement with the elemental composition data, suggest a good quality WS 2 NTs.…”

“…[ 30–33 ] Their scalable production was reported in 2009 [ 34 ] and further revised recently, extending the acquired experience to the synthesis of MoS 2 INTs in pure phase and significant amounts. [ 35 ] The availability of these nanotubes has raised a great interest in the scientific community and has enabled the wide investigation of their properties. The layered structure of WS 2 INTs supports inert van der Waals surface combined with excellent optoelectrical properties [ 36–40 ] making WS 2 NTs suitable candidates for the next generation of electronic and optoelectronic applications.…”

Interaction of Poly L-Lactide and Tungsten Disulfide Nanotubes Studied by In Situ X-ray Scattering during Expansion of PLLA/WS2NT Nanocomposite Tubes

“…The two main steps of the reaction, the growth of suboxide whiskers and their sulfurization, occur in the same reactor at elevated temperatures (greater than 800 °C) and under the same H 2 S/H 2 /N 2 gas flow (N 2 is a carrier gas), following each other by a self-control mechanism. A detailed description of the reaction route and growth mechanism of WS 2 NTs was reported earlier [ 51 ], followed by newer insights [ 62 ].…”

Poly(L-lactic acid) Reinforced with Hydroxyapatite and Tungsten Disulfide Nanotubes

“…INT-WS 2 with diameters between 30 and 150 nm and lengths between 1 and 20 micrometers were synthesized using a published procedure [39]. Briefly, the precursor nanoparticles of tungsten trioxide, grow into high aspect ratio tungsten suboxide nanowhiskers under a mild reducing atmosphere at 840 • C. Subsequent sulfurization of the nanowhiskers results in hollow WS 2 nanotubes.…”