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 <i>via</i> 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 <i>via</i> VGS was the instability of the <i>in situ</i> 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 <i>via</i> 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.