Structural Evolutions of Vertically Aligned Two-Dimensional MoS₂ Layers Revealed by in Situ Heating Transmission Electron Microscopy

Benefiting from a large density of layer edges exposed on the surface, vertically aligned two-dimensional (2D) molybdenum disulfide (MoS₂) layers have recently harvested excellent performances in the field of electrochemical catalysis and chemical sensing. With their increasing versatility for high-temperature, demanding applications, it is vital to identify their thermally driven structural and chemical stability, as well as to clarify its underlying principle. Despite various ex situ and in situ characterizations on horizontally aligned 2D MoS₂ layers, the direct in situ heating of vertically aligned 2D MoS₂ layers and the real-time observation of their near-atomic-scale dynamics have never been approached, leaving their thermal stability poorly understood. Moreover, the geometrical advantage of the surface-exposed vertically aligned 2D MoS₂ layers is anticipated to unveil the structural dynamics of interlayer van der Waals (vdW) gaps and its correlation with thermal energy, unattainable with 2D MoS₂ layers in any other geometry. Herein, we report a comprehensive in situ heating TEM study on cleanly transferred, vertically aligned 2D MoS₂ layers up to 1000 °C. Several striking phenomena were newly observed in the course of heating: (1) formation and propagation of voids between the domains of vertical 2D MoS₂ layers with distinct grain orientations starting at ∼875 °C; (2) subsequent decompositions of the 2D MoS₂ layers accompanying a formation of Mo nanoparticles at ∼950 °C, a temperature much lower than the melting temperature of their bulk counterpart; and (3) initiation of decomposition from the surface-exposed 2D layer vertical edge sites, congruently supported by molecular dynamics (MD) simulation. These new findings will offer critical insights into better understanding the thermodynamic principle that governs the structural stability of general vdW 2D crystals as well as providing useful technological guidance for materials design and optimization in their potential high-temperature applications.