Liquid phase exfoliation of tungsten diselenide nanosheets and their electrochemical properties

The electrochemical performance of two-dimensional (2D) materials, particularly transition metal dichalcogenides (TMDs), is governed by their structural and chemical properties. Large-scale, high-yield production methods must ensure both nanosheet quality and electrochemical suitability for practical applications in energy storage, electrocatalysis, and electrochemical adsorption. This thesis investigates liquid-phase exfoliation as a scalable synthesis method, focusing on optimising solvent choice and exfoliation power to enhance structural integrity and functionality of TMD nanosheets. Given the limitations of graphene in electrochemical applications, alternative materials such as MoS2, MoSe2, and WSe2 are explored. A comparative study of these TMDs highlights distinct caesium ion removal mechanisms: MoS2 primarily stores charge via pseudocapacitive intercalation (0.78 C g-1, with 2.6% from EDL formation), MoSe2 relies on electric double-layer (EDL) formation (60% of 0.54 C g-1), while WSe2 exhibits a hybrid behaviour (0.77 C g-1, with 14% from EDL). These findings support their potential as efficient and tuneable materials for electrochemical separation processes. Among them, WSe2 was selected for in-depth study due to its balanced charge storage and promising electrocatalytic properties. Structural characterisation revealed that increasing exfoliation power (90.4 to 814 W) led to higher selenium vacancy formation, increased oxide content (44.3 to 53.9%), which in turn influenced HER and OER performance. High-power exfoliated WSe2 showed diminished HER activity due to W–Se bond loss but enhanced OER activity, especially in alkaline media. Proton interaction with selenium vacancies also enhanced capacitive behaviour under acidic conditions. Recognising the oxidative effects of N-methyl-2-pyrrolidone (NMP), a greener solvent mixture of isopropyl alcohol and deionised water (IPA/DI) was employed, reducing oxidation and preserving WSe2 structure. Over time, IPA/DI-exfoliated WSe2 nanosheets exhibited superior HER performance, achieving overpotentials as low as -0.206 V vs. RHE at -10 mA cm-2, comparable to platinum. This study provides practical guidance for tuning WSe2 nanosheet properties via exfoliation and storage conditions, advancing their application in electrocatalysis and environmental remediation.