Salt-assisted synthesis of high phase-purity metastable 1T'-phase group VIB transition metal dichalcogenides.

With the increasing progress in the emerging field of phase engineering of nanomaterials, transition metal dichalcogenides (TMDs) with metastable phases have attracted extensive research interest owing to their unique physicochemical properties and promising potential for various applications. Unlike their semiconducting counterparts, metastable 1T'-phase group VIB TMDs exhibit distinctive metallic behavior, making them highly promising for clean energy electrocatalysis (for example, hydrogen evolution), high-performance electronics and superconducting devices. However, conventional synthetic strategies usually suffer from low yields, poor phase purity, small crystal size and harsh experimental conditions, hindering their fundamental research and practical applications. Here we describe a salt-assisted method for the controlled preparation of various metastable 1T'-phase group VIB TMDs with high crystalline quality and high phase purity. In this context, 'salt-assisted' refers to the co-annealing of commercially available 2H-phase TMDs, chalcogen powders (for example, S or Se) and alkali metal salts (for example, K2C2O4·H2O or K2CO3). When heated in a tube furnace under an H2/Ar atmosphere, these alkali metal salts trigger a phase transformation of TMDs from the thermodynamically stable 2H phase to the metastable 1T' phase. This Protocol describes the details of experimental procedures to prepare a series of 1T'-TMDs, that is, MoS2, MoSe2, WS2, WSe2, MoS2xSe2(1-x) and WS2xSe2(1-x). It takes ~37 h 20 min and yields robust 1T'-TMD crystals with their size of up to hundreds of micrometers at a gram scale (up to ~2 g per batch), overcoming the longstanding synthetic bottlenecks and paving the way for advanced fundamental studies and the exploration of practical applications.