Transition metal dichalcogenide semiconductors formed by monolayers of composition TX₂, with T and X being a transition metal element and a chalcogen element respectively. In particular, transition metal sulfides, such as MoS2 and WS2 have become research hotspots in recent times. Among the different metal disulfide nanostructures, WS2 inorganic fullerenes are usually obtained via a solid−gas reaction at a relatively high temperature from an oxide nanoparticles powder (WO3) exposed to a H2/H2S gas mixture. The formation mechanism involved here has been widely studied as it represents a way of producing highly sought-after nanoparticles directly related to the shape and size of the oxide nanoparticles. Previous publications have revealed that confinement can limit and/or orient nanoparticles growth. Therefore, composite materials made of nanostructured dichalcogenides located in a porous matrix have been also prepared; resulting in interesting properties for various applications. However, a thorough review of published literature reveals that very few studies have explored the possibility of metal dichalcogenide synthesis in a confined environment with formation of inorganic nanotubes and fullerenes.
To bridge this deficiency, researchers from the Institute of Materials Science of Mulhouse at Upper-Alsace University (UHA) and Institute of Physics and Chemistry of Materials Strasbourg at University of Strasbourg in France: Dr. Julien Kiener, Dr. Maria Girleanu, Professor Ovidiu Ersen and Dr. Julien Parmentier investigated a novel growth mechanism for WS2 nanostructures with well-defined shapes (nanocages or nanotubes), induced by a confinement effect in a mesoporous structure. Their work is currently published in the research journal, Crystal Growth & Design.
In their approach, the research team synthesized WS2/ C Nanocomposites where the carbon matrix has a honey-comb mesoporous structure. This synthesis was based on the soft-template route mixing an amphiphilic surfactant as a pore template, a phenolic resin as a carbon precursor and a W-based salt as a WS2 precursor. Samples were obtained in a single step (one-pot procedure) thanks to the evaporation induced self-assembly technique followed by heat-treatment in H2S atmosphere as depicted in related literature. The researchers then characterized the resulting samples via XRD, Thermogravimetric analysis and TEM analyses – among other techniques.
Interestingly, at the difference of former mechanisms described in literature, the unidirectional mesoporosity associated by the specificity of the one-pot synthesis method was reported to lead to original WS2 curved nanostructures (nanocages, nanotubes) within the porosity. The proposed gas-solid growth mechanism is based on successive reactions of H2S and W-based gaseous species on the dangling bond of the existing WS2 layers formed during the previous nucleation step. Further, carbon was also reported to contribute to the reduction of the oxidation state of W, thus simplifying the process of the formation of WS2 nanometric phases.
In summary, the study presented a unique preparation of original nanostructures such as nanometric slabs, nanoribbons, as well as WS2 individual inorganic nanocages and nanotubes, through a mechanism involving the confinement of reactive species within mesopores of a carbon matrix. Remarkably, it was revealed that lamellar growth occurred for nano-slab well oriented along the longitudinal axis of the mesopores (e.g., nanoribbons in cylindrical pores), whereas for the others, owing to their low curvature energy, their growth occurred by their rolling along the mesopore surface with formation of curved nanolayers, which subsequently lead to inorganic nanocages (with ellipsoidal shape) or nanotubes.
Julien Kiener, Maria Girleanu, Ovidiu Ersen, Julien Parmentier. Direct Insight into the Confinement Effect of WS2 Nanostructures in an Ordered Carbon Matrix. Crystal Growth Design 2020, volume 20, page 2004−2013.