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

Abstract

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 (–OP) 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.