Anion Extraction-Induced Polymorph Control of Transition Metal Dichalcogenides.

Controlled phase conversion in polymorphic transition metal dichalcogenides (TMDs) provides a new synthetic route for realizing tunable nanomaterials. Most conversion methods from the stable 2H to metastable 1T phase are limited to kinetically slow cation insertion into atomically thin layered TMDs for charge transfer from intercalated ions. Here, we report that anion extraction by the selective reaction between carbon monoxide (CO) and chalcogen atoms enables predictive and scalable TMD polymorph control. Sulfur vacancy, induced by anion extraction, is a key factor in molybdenum disulfide (MoS) polymorph conversion without cation insertion. Thermodynamic MoS-CO-CO ternary phase diagram offers a processing window for efficient sulfur vacancy formation with precisely controlled MoS structures from single layer to multilayer. To utilize our efficient phase conversion, we synthesize vertically stacked 1T-MoS layers in carbon nanofibers, which exhibit highly efficient hydrogen evolution reaction catalytic activity. Anion extraction induces the polymorph conversion of tungsten disulfide (WS) from 2H to 1T. This reveals that our method can be utilized as a general polymorph control platform. The versatility of the gas-solid reaction-based polymorphic control will enable the engineering of metastable phases in 2D TMDs for further applications.