The Dynamic Interaction of Surfactants with Colloidal Molybdenum Disulfide Nanosheets Calls for Thermodynamic Stabilization by Solvents

Abstract

Top-down liquid-phase exfoliation (LPE) and bottom-up hot-injection synthesis are scalable methods to produce colloids of two-dimensional (2D) van der Waals (vdW) solids. Generally thought off as two entirely different fields, we show that similar stabilization mechanisms apply to colloids of molybdenum disulfide (MoS2) produced by both methods. By screening the colloidal stability of MoS2 produced in a hot-injection synthesis in a wide range of solvents, we observe that colloidal stability can be understood based on solution thermodynamics, wherein matching the solubility parameter of solvent and nanomaterial maximizes colloidal stability. Identical to MoS2 produced through LPE, optimal solvents to disperse MoS2 produced from the bottom-up have similar solubility parameters of ≈22 MPa1/2 and include aromatic solvents with polar functionalities, such as o-dichlorobenzene, and polar aprotic solvents, such as N,N-dimethylformamide. We further complemented our findings by nuclear magnetic resonance (NMR) spectrscopy, highlighting that organic surfactants, such as oleylamine and oleic acid, have a minimal affinity toward the nanocrystal surface and engage in a highly dynamic adsorption/desorption equilibrium. We thus conclude that hot injection yields MoS2 colloids with comparable surfaces as those produced by LPE. These similarities might offer the prospect of using established procedures developed for LPE nanomaterials to postprocess colloidally synthesized dispersions of 2D colloids as processable inks.

Colloidal 2D Mo1−xWxS2 nanosheets: an atomic- to ensemble-level spectroscopic study

“…UV-Vis absorbance spectra were acquired on an Agilent Cary 5000, using a quartz cuvette (quartz glass high performance QS 200–2500 nm by Hellma) with a path length of 1 cm (optical density below 0.2 at 350 nm) All samples were dispersed in o -DCB for maximum colloidal stability. 29…”

Two-Dimensional MoS2 Nanosheets Derived from Cathodic Exfoliation for Lithium Storage Applications

“…On the other hand, top-down methods relying on the exfoliation of bulk MoS 2 crystals appear to be particularly convenient for accessing stand-alone, processable 2D MoS 2 flakes in substantial amounts [ 19 , 20 ]. For example, direct liquid-phase exfoliation of MoS 2 in proper organic solvents [ 21 ] or water/surfactant solutions [ 22 , 23 ], prompted by ultrasound or shear forces, is a common strategy to attain stable colloidal dispersions of MoS 2 NSs in considerable quantities. Nonetheless, this method typically suffers from low exfoliation yields and degrees (NSs comprising several layers rather than single/few layers), thus limiting their practical application potential.…”