The structure analysis and chemical properties probing during recycling processes of transition metal dichalcogenides exfoliation

Transition metal dichalcogenides (TMDs) have attracted much attention in electrochemistry due to their outstanding properties; however, there is a limit to the production of TMDs samples on a large scale. In this work, we have demonstrated the recycling process of TMDs materials using WSe2 as a representative of the TMDs family. WSe2 nanoflakes were synthesised by solvent-assisted liquid phase exfoliation through sonication of "bulk" WSe2. It has been found that the unexfoliated WSe2 can be recycled up to 6 times, this improve the production yield. The physical and chemical properties of the exfoliated WSe2 nanoflakes were investigated during the recycling processes. Interestingly, the recycling processes (number of recycling times) directly affect the flake sizes, and surface chemistry of the exfoliated samples. The obtained flake size decreases from about 1 mu m to less than 200 nm (from 1st to 7th exfoliation), the diminished of sample thickness has also been found. Besides, the local structure of WSe2 remains unchanged, showing the W4+ oxidation state of WSe2, which to the best of our knowledge. The applicability of the as-recycled samples were then demonstrated as bi-functional electrode materials: (i) supercapacitor, (ii) and electrocatalyst (for hydrogen evolution reaction). From 1st to 7th exfoliation, the electrochemical performance improves about 1-fold (capacitance increase from 3 to 6 F g(+1), and the overpotential at 10 mA cm(-2) improves from -0.67 to -0.42 vs. RHE). This study provides an alternative way to improve the synthesis method, which can not only utilise "waste" but also recover the production yield of TMDs leading to the continued development of 2D materials, and related families.