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

초록

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 00000000 00000000 00000000 00000000 11111111 00000000 11111111 00000000 00000000 00000000 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. Spectroscopic investigation of tungsten dichalcogenide nanosheets is reported, where the differentiated metal-ligand coordination geometry influenced by the choice of appropriate ligands is a pivotal factor for the phase-selective formation.

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