Cation-Controlled Electrocatalytical Activity of Transition-Metal Disulfides

The nanostructured layered transition-metal dichalcogenides are highly promising materials for substitution of platinum metals in electrocatalysis. These layered materials are usually exfoliated with intercalation of Li from organolithium compounds. Alkali metal intercalation leads not only to exfoliation and increased surface area but also, even more importantly, to conversion of naturally occurring 2H (trigonal prismatic) semiconducting phase of MoS2/WS2 to 1T octahedral conducting phase, which also exhibits better electrocatalytical performance. Surprisingly, even for the most broadly studied members like MoS2 and WS2, systematic work on their exfoliation is missing. We present a detailed study of MoS2 and WS2 exfoliation by intercalating cations of variable size (Li, Na, K, Rb, Cs). Alkali metal naphtalenides that are capable of producing single-layer exfoliated dichalcogenides in quantitative yield were used. The results show strong dependence of dichalcogenide exfoliation on cation ionic radii. The cation has also a significant influence on the 1T/2H phase ratio in the resulting exfoliated materials. The overpotential and Tafel slopes directly correlate with the concentration of 1T phase. Our findings have broad fundamental implications as well as practical applications because by choosing the intercalating ion, one can tailor the exfoliation procedure.