Engineering polymorphs in colloidal metal dichalcogenides: precursor-mediated phase control, molecular insights into crystallisation kinetics and promising electrochemical activity

Controlling the crystal phase in layered transition metal dichalcogenides (TMDs) is critical for their diverse, flexible applications. However, due to the thermodynamic stability of 2H over other polymorphs, fine synthesis control over polymorphism in TMDs is challenging, restricting the entire range of characteristics associated with other polymorphs. Herein, we present a solution-based crystal phase engineering approach for layered transition metal disulphide nanosheets by modulating the reactivity of the molecular precursors. By tuning precursor-ligand chemistry, 2H, 1T ' and polytypic MoS2 and WS2 are synthesised. The flexibility to selectively modify the reactivity of S and metal precursors allowed control over the proportion of specific phases in synthesised nanosheets. The formation of 1T ' is facilitated by the highly reactive metal and S source, whereas less reactive sources lead to the formation of thermodynamically stable 2H. The electrocatalytic properties of the synthesised TMDs were examined for the oxygen reduction reaction. The polytypic MoS2 comprising a mix of 2H-1T ' displayed the most positive potential of 0.82 V (vs. RHE). The comprehensive mechanistic interpretation of the chemical transformations provided in this study will be instrumental in designing scalable solution-based pathways for phase engineering in layered transition metal dichalcogenides. Furthermore, this synthesis approach has the potential to be extended to various TMD compositions, enabling exquisite control over polymorphism in TMDs.