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 "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.