Phase Engineering of Transition Metal Dichalcogenides with Unprecedentedly High Phase Purity, Stability

A technology has been developed to improve the performance of the’transition metal chalcogen compound (TMD)’, which is popular as a next-generation hydrogen generation catalyst using liquid alkali metal. A new synthetic method that allows liquid metal to penetrate into the catalyst structure can quickly and easily increase the electrical conductivity, which has been pointed out as a weakness of the catalyst.

A research team, jointly led by Professors Hyesung Park, Guntae Kim, and Sang Kyu Kwak in the School of Energy and Chemical Engineering at UNIST, has developed a synthetic method that converts the transition metal chalcogen compound into a metal phase (1T phase) using the ‘alkali molten metal (liquid metal such as molten metal) intercalation method.’ The’transition metal chalcogen compound’, which is attracting attention as a catalyst for generating hydrogen, improves its performance as the electrical conductivity increases. It is a technology that converts a semiconductor phase into a metal phase with excellent electrical conductivity in a short time using a simple synthesis method.

The transition metal chalcogen compound (hereinafter, a chalcogen compound) is a material in which a metal element such as tungsten (W) or molybdenum (Mo) and a chalcogen element such as sulfur (S) are combined. Due to its low price and high durability, it is being studied as a catalyst for “water electrolysis” (a reaction that produces hydrogen from water) that will replace platinum (Pt). However, at room temperature, the electrical conductivity, which is a measure of the performance of the catalyst, falls. In this material, a part having a semiconductor property and a part having a metal property coexist in one material, because it exists as a semiconductor phase with low electrical conductivity at room temperature. There is a method of synthesizing to have a metallic phase, but it takes a long time, and there is a limit in that the synthesized material returns to a semiconductor-like material.

The joint research team succeeded in synthesizing a “metallic chalcogen compound” in 1 hour (from 48 to 72 hours) by inserting a liquid alkali metal into a transition metal chalcogen compound using capillary phenomenon. In this case, the alkali metal serves to supply electrons necessary for the transition metal chalcogen compound to be converted into a metal phase. Liquid alkali metal is well transferred to the inside of the chalcogen compound because it uses the ‘capillary phenomenon,’ in which liquid is automatically sucked into the thin tube. In the case of the thus synthesized transition metal chalcogen compound, the ratio of the metal phase to the total compound was as high as 92%.

“In the case of the existing synthesis method, a metallic transition metal chalcogen compound is prepared over 2 to 3 days, and the synthesis method developed this time is short and simple,” says Sanghyeon Park, first author of the study. “High phase purity of 92% or more was confirmed through X-ray photoelectron analysis.”

In particular, the metal-phase transition metal chalcogen compound synthesized this time has the advantage of excellent stability. Even with high heat and strong light, the metal phase remained unchanged into a semiconductor phase. Through theoretical analysis, the research team also revealed the cause of the stable maintenance of the metal phase. In the synthesis process, it was found that the bonding between the alkali metal and the chalcogen material lowered the energy barrier required to change the semiconductor phase into a metal phase and maintained the electronic structure. In addition, as a result of applying the newly synthesized chalcogen compound to an actual water electrolysis system, it showed excellent performance even in operation over 100 hours.

“We have discovered a new method for synthesizing a transition metal chalcogen compound, which is drawing attention as a next-generation hydrogen generation catalyst,” says Professor Park. “This research not only provided clues to identify the physical properties of the two-dimensional material, but is also expected to be of great help in the development of a hydrogen generation catalyst by utilizing the properties of the metal-phase transition metal chalcogen compound.”

The findings of this research have been published in Advanced Materials on July 6, 2020. This study has been supported by the Individual Basic Science & Engineering Research Program and the Mid-Career Researcher Program through the National Research Foundation of Korea (NRF).

Journal Reference

Sanghyeon Park, Changmin Kim, Sung O. Park, et al., “Phase Engineering of Transition Metal Dichalcogenides with Unprecedentedly High Phase Purity, Stability, and Scalability via Molten‐Metal‐Assisted Intercalation,” Advanced Materials, (2020).

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