Misfit Layered\nCompounds: Insights into Chemical,\nKinetic, and Thermodynamic Stability of Nanophases

ConspectusCompounds with layered structures\n(2D-materials), like transition\nmetal-dichalcogenides (e.g., MoS<sub>2</sub>), attracted a great deal\nof interest in the scientific community in recent years. This interest\ncan be attributed to their unique lamellar structure, which induces\nlarge anisotropy in their physicochemical properties. Furthermore,\nowing to the weak van der Waals interaction between the layers, they\ncan be cleaved along the <i>a–b</i> plane, which\nallows fabricating single layers with physical properties entirely\ndifferent from the bulk material. Moreover, stacking layers of different\n2D-materials on top of each other has led to a wealth of new observations,\nfor instance, by twisting two layers with respect to each other and\nproducing Moiré lattice. Another outstanding property of inorganic\nlayer compounds is their tendency to form nanotubes, reported first\n(for WS<sub>2</sub>) many years ago and subsequently from many other\nlayered compounds.Among the 2D-materials, misfit layer compounds\nmake a special class\nwith an incommensurate and nonstoichiometric lattice made of an alternating\nlayer with rocksalt structure, like LaS (<i>O</i>) and a\nlayer with hexagonal structure, like TaS<sub>2</sub> (<i>T</i>). The lack of lattice commensuration between the two slabs leads\nto a built-in strain, which can be relaxed via bending. Consequently,\nnanotubes have been produced from numerous MLC compounds over the\npast decade and their structure was elucidated.Owing to their\nlarge surface area, nanostructures are generally\nmetastable and tend to recrystallize into microscopic crystallites\nvia different mechanisms, like Ostwald ripening, or chemically decompose\nand then recrystallize. The stability of nanostructures at elevated\ntemperatures has been investigated quite scarcely so far. In this\nperspective, electron microscopy as well as synchrotron-based X-ray\nabsorption and reflection techniques were used to elucidate the chemical\nselectivity and decomposition routes of rare-earth based MLC nanotubes\nprepared at elevated temperatures (800–1200 °C).As for the chemical selectivity, entropic effects are expected\nto dictate the random distribution of the chalcogen atoms on the anion\nsites of the MLC nanotubes at elevated temperatures. Nonetheless,\nthe sulfur atoms were found to bind exclusively to the rare-earth\natom (Ln = La, Sm) of the rocksalt slab and the selenium to the tantalum\nof the hexagonal TX<sub>2</sub> slab. This uncommon selectivity was\nnot found in other kinds of nanotubes like WSe<sub>2<i>x</i></sub>S<sub>2(1–<i>x</i>)</sub>. In other series\nof experiments, the lack of utter symmetry in the multiwall nanotubes\nleads to exclusions of certain X-ray (0<i>kl</i>) reflections,\nwhich was used to distinguish them from the bulk crystallites. The\ntransformation of Ln-based MLC nanotubes into microscopic flakes was\nfollowed as a function of the synthesis temperature (800–1200\n°C) and the synthesis time (1–96 h). Furthermore, sequential\nhigh-temperature transformations of the (<i>O-T</i>) lattice\ninto (<i>O-T-T</i>) and finally (<i>O-T-T-T</i>) phases via deintercalation of the LnS slab was observed. This autocatalytic\nprocess is reminiscent of the deintercalation of alkali atoms from\ndifferent layered structure materials. Annealing at higher temperatures\nand for longer periods of time eventually leads to the decomposition\nof the ternary MLC into binary metal-sulfide phases, as well as partial\noxidation of the product. This study sheds light on the complex mechanism\nof high-temperature chemical stability of the nanostructures.