Misfit Layered Compounds: Insights into Chemical, Kinetic, and Thermodynamic Stability of Nanophases

Abstract

Metrics & MoreArticle Recommendations * sı Supporting Information CONSPECTUS: Compounds with layered structures (2D-materials), like transition metal-dichalcogenides (e.g., MoS 2 ), attracted a great deal of interest in the scientific community in recent years. This interest can be attributed to their unique lamellar structure, which induces large anisotropy in their physicochemical properties. Furthermore, owing to the weak van der Waals interaction between the layers, they can be cleaved along the a−b plane, which allows fabricating single layers with physical properties entirely different from the bulk material. Moreover, stacking layers of different 2Dmaterials on top of each other has led to a wealth of new observations, for instance, by twisting two layers with respect to each other and producing Moirélattice. Another outstanding property of inorganic layer compounds is their tendency to form nanotubes, reported first (for WS 2 ) many years ago and subsequently from many other layered compounds.Among the 2D-materials, misfit layer compounds make a special class with an incommensurate and nonstoichiometric lattice made of an alternating layer with rocksalt structure, like LaS (O) and a layer with hexagonal structure, like TaS 2 (T). The lack of lattice commensuration between the two slabs leads to a built-in strain, which can be relaxed via bending. Consequently, nanotubes have been produced from numerous MLC compounds over the past decade and their structure was elucidated.Owing to their large surface area, nanostructures are generally metastable and tend to recrystallize into microscopic crystallites via different mechanisms, like Ostwald ripening, or chemically decompose and then recrystallize. The stability of nanostructures at elevated temperatures has been investigated quite scarcely so far. In this perspective, electron microscopy as well as synchrotronbased X-ray absorption and reflection techniques were used to elucidate the chemical selectivity and decomposition routes of rareearth based MLC nanotubes prepared at elevated temperatures (800−1200 °C).As for the chemical selectivity, entropic effects are expected to dictate the random distribution of the chalcogen atoms on the anion sites of the MLC nanotubes at elevated temperatures. Nonetheless, the sulfur atoms were found to bind exclusively to the rare-earth atom (Ln = La, Sm) of the rocksalt slab and the selenium to the tantalum of the hexagonal TX 2 slab. This uncommon selectivity was not found in other kinds of nanotubes like WSe 2x S 2(1−x) . In other series of experiments, the lack of utter symmetry in the multiwall nanotubes leads to exclusions of certain X-ray (0kl) reflections, which was used to distinguish them from the bulk crystallites. The transformation of Ln-based MLC nanotubes into microscopic flakes was followed as a function of the synthesis temperature (800− 1200 °C) and the synthesis time (1−96 h). Furthermore, sequential high-temperature transformations of the (O-T) lattice into (O-T-T) and finally (O-T-T-T) phases via deinter...