Unveiling the ligand-mediated phase engineering mechanism in two-dimensional transition metal chalcogenides through coordination geometry control

While metastable metallic phases of group-VI transition metal dichalcogenide (TMDC) nanosheets exhibit intriguing and unprecedented characteristics, the development of reliable synthetic methodologies, particularly in direct solution-based synthesis, remains a challenge due to the lack of understanding of molecular leverage of the metal-ligand coordination geometry for obtaining metastable phases in two-dimensional (2D) TMDCs. Here, we describe an effective solution-based approach for directly synthesizing metastable metallic phases by unveiling the criterion for phase-selective formation, using appropriate ligands. Specifically, metallic 1T′-WSe₂ and 1T-WS₂ nanosheets were obtained with trioctylphosphine oxide, whereas 2H-WSe₂ and 2H-WS₂ nanosheets were formed using oleylamine. Spectroscopic analysis, including X-ray pre-edge absorption, revealed that phosphine oxide ligands (-O = P) induce the distorted octahedral metal-ligand geometry, followed by the phase-selective formation of metallic 1T′ and 1T phases. Meanwhile, amine ligands (-NH₂), accompanied by the trigonal prismatic metal-ligand geometry, exclusively lead to the production of 2H phases. This strategy was applied using hexadecylamine and triphenylphosphine oxide to produce 2H and 1T′ phases, respectively. As a proof-of-concept study, metallic 1T′-WSe₂ shows enhanced hydrogen evolution activity with long-term durability. This strategy, controlling the metal-ligand coordination geometry by the choice of suitable ligands, offers a new guideline for securing metastable phases in 2D TMDCs.