Systematic investigation on the surfactant-assisted liquid phase exfoliation of MoS2 and WS2 in water for sustainable 2D material inks

Abstract

MoS₂ and WS₂ have gathered significant attention due to their interesting and tunable properties and their wide range of applications. Liquid phase exfoliation (LPE) is a common and facile method to prepare 2D MoS₂ and WS₂. Currently, the principally employed solvents for LPE of MoS₂ and WS₂ are expensive, toxic and have high boiling points. These drawbacks encourage to find more sustainable alternatives to the liquid medium used for the preparation of 2D material inks. Water would be the best option, but surfactants are necessary for LPE in water, since MoS₂ and WS₂ are hydrophobic. Organic molecules with amphoteric character such as sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS) and sodium hexyl sulfonate (SHS) are selected as suitable candidates for the role of surfactant. However, the study of these surfactants used in LPE is barely systematically reported. In this work, we present a detailed investigation on the impact of these surfactants on the LPE of MoS₂ and WS₂, which are representatives of TMDs. By characterizing and qualifying the products from average number of layers, we find that all the surfactants work efficiently to exfoliate MoS₂ and WS₂ into few layers, and SHS stabilizes the 2D layers better than the other two surfactants. However, in terms of yield and relative surfactant concentration, we don’t identify a real trade-off between maximized quantity of exfoliated materials and minimized surfactant concentration, which prompts to select the colloidal ink based on the specific further needs for processing.