Phase Engineering of Transition Metal Dichalcogenides for Energy Storage and Conversion

Phase engineering of nanomaterials (PEN) has attracted increasing research interest recently. Through phase engineering, the performance of nanomaterials could be boosted, which is broadly studied in the fields of catalysts, batteries, semiconductors, and biomedicines. Despite these, great challenges still remain in the synthesis of nanomaterials with specific phases for the study of their phase-dependent properties and applications.

Transition metal dichalcogenides (TMDs), an important family of two-dimensional materials with extensive applications, have attracted great attention due to their polymorphs with unique physicochemical properties. Until now, researchers have made huge progress in exploring novel phases and improving phase purity of TMDs. In particular, the phase transformation might be an effective way to prepare TMDs with unconventional phases. In this thesis, phase engineering of transition metal dichalcogenides is used to achieve excellent catalytic and battery performance.

In Chapter2, hexagonal-phase tungsten nitride nanotubes (NTs) are synthesized based on the 2H-phase WS₂ NTs and evaluated as the electrocatalyst for HER. Raman spectra reveal that after the nitridation of WS2 NTs, only one peak at ~246 cm^-1 appears in the range of 100-500 cm^-1, which is consistent with the previously reported Raman results of tungsten nitrides. High-resolution X-ray photoelectron spectroscopy spectra demonstrate that theWS₂ NTs are completely nitridated since the S signal cannot be detected and N signal emerges correspondingly. Importantly, the tube structure is well maintained after nitridation, as proven by high-resolution transmission electron microscope. Simulated X-ray diffraction (XRD) signals from the electron diffraction patterns of the tungsten nitride NTs can be well indexed to the hexagonal-phase WN_1.5. As a proof-of-concept application, the synthesized WN_1.5 NTs exhibit excellent catalytic performance for hydrogen evolution reaction. In addition, this material is found to exhibit semimetal-like behavior.

In Chapter 3,phase-dependent energy storage properties of TaS₂ are studied. When used in lithium-ion batteries, the 1T phase exhibits a specific capacity of~430 mA h g^-1, which is much higher than that of the 2H phase (~340mA h g^-1). However, when used in sodium ion batteries, the 1T phase only exhibits ~150 mA h g^-1, while the 2H phase can achieve a higher value of ~170 mA h g^-1. The phase-dependent energy storage performance of TaS₂ originates from the different reaction mechanisms, which are reflected in the discharge-charge curves, the cyclic voltammetry curves, as well as the in situ XRD patterns.

In Chapter 4,perspectives on phase engineering of TMDs for enhanced energy storage and conversion performance are further proposed.