Effect of Surfactant Choice and Concentration on the Dimensions and Yield of Liquid-Phase-Exfoliated Nanosheets

Abstract

Liquid phase exfoliation has progressed in recent years to become a common method of production for 2D materials. During exfoliation, surfactants can be used to stabilize the nanosheets against reaggregation. Here, using WS2 as a model system, we explore the effect of varying surfactant type and concentration on the yield and dimensions of the exfoliated nanosheets. For ionic surfactants, the mass of nanosheets produced was constant for low surfactant concentrations but decreased sharply for surfactant concentrations above ~10 mM, regardless of surfactant. Very similar surfactant-concentration dependence was observed for both nanosheet length and thickness. Contrary to previous reports, this data implies that the optimum surfactant concentration is not linked to the critical micelle concentration. In addition, we found that surfactant concentrations as low as 0.07 mM yielded stable nanosheet dispersions with zeta potential above 40 mV. By decoupling the exfoliation and stabilization effects of the surfactant, we have shown that it is the (de)stabilization process, rather than the exfoliation process, which links nanosheet concentration, size and thickness to surfactant concentration.

Impact of Pretreatment of the Bulk Starting Material on the Efficiency of Liquid Phase Exfoliation of WS2

“…However, the standard errors of the averages, which is the relevant quantity in this case, can be assessed with reasonable accuracy. The overall error of the determination of the average length and thickness from optical extinction spectra was recently evaluated in context of reference [ 25 ] and found to be 6% for <L> and 13% for <N>.…”

“…One downside of LPE is that exfoliated stock dispersions will always be polydisperse with a broad distribution in nanosheet size [ 25 , 26 ]. Centrifugation can be applied to narrow the size and thickness distributions [ 27 ].…”

“…While liquid-exfoliated nanosheets are considered widely defect-free on the basal plane [ 34 , 36 ], a number of groups have found evidence for basal plane defects in the case of graphene [ 37 , 38 , 39 ]. In addition, it is typically observed that nanosheets thinned down to the monolayer (ML) limit are comparably small in their lateral dimensions (~100–200 nm for graphene, <50 nm for TMDs), despite of many attempts to optimize the exfoliation process [ 25 , 40 , 41 , 42 , 43 , 44 ]. In general, it is found that thin sheets tend to be small, while large sheets tend to be thicker [ 8 , 26 , 45 ].…”

Site‐Selective Oxidation of Monolayered Liquid‐Exfoliated WS2 by Shielding the Basal Plane through Adsorption of a Facial Amphiphile

“…No further size selection was performed prior to reaction with chloroauric acid. As such, the WS 2 samples are polydisperse in size with a broad distribution of lateral dimensions ranging from 50–400 nm and thicknesses from 1–40 layers . In such samples, monolayer contents are very low (<0.5 %) .…”

“…Ionic surfactants are widely used in stabilizing colloidal nanomaterials such as carbon nanotubes, graphene, h‐BN, or TMDs . Generally, they can be roughly divided into facial and linear amphiphiles.…”