A critical assessment of the role of ionic surfactants in the exfoliation and stabilization of 2D nanosheets: The case of the transition metal dichalcogenides MoS2, WS2 and MoSe2

Bárbara Abreu; Bernardo Almeida; Pedro Ferreira; Ricardo M F Fernandes; Diana M Fernandes; Eduardo F Marques

doi:10.1016/j.jcis.2022.06.097

A critical assessment of the role of ionic surfactants in the exfoliation and stabilization of 2D nanosheets: The case of the transition metal dichalcogenides MoS₂, WS₂ and MoSe₂

J Colloid Interface Sci. 2022 Nov 15:626:167-177. doi: 10.1016/j.jcis.2022.06.097. Epub 2022 Jun 24.

Authors

Bárbara Abreu¹, Bernardo Almeida², Pedro Ferreira², Ricardo M F Fernandes², Diana M Fernandes³, Eduardo F Marques⁴

Affiliations

¹ CIQUP - Centro de Investigação em Química da Universidade do Porto, Institute of Molecular Sciences (IMS), Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre s/n, Porto, 4169-007, Portugal; REQUIMTE-LAQV, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre s/n, Porto, 4169-007, Portugal. Electronic address: barbara.abreu@fc.up.pt.
² CIQUP - Centro de Investigação em Química da Universidade do Porto, Institute of Molecular Sciences (IMS), Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre s/n, Porto, 4169-007, Portugal.
³ REQUIMTE-LAQV, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre s/n, Porto, 4169-007, Portugal.
⁴ CIQUP - Centro de Investigação em Química da Universidade do Porto, Institute of Molecular Sciences (IMS), Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre s/n, Porto, 4169-007, Portugal. Electronic address: efmarque@fc.up.pt.

PMID: 35785602
DOI: 10.1016/j.jcis.2022.06.097

Abstract

Transition metal dichalcogenides (TMDs), like other two-dimensional layered materials beyond graphene, have gained enormous interest in recent years owing to their distinct electronic and optical properties, and potential applicability in areas such as sensing, nanoelectronics and catalysis. Surfactant-assisted exfoliation is commonly used to prepare aqueous dispersions of TMD nanosheets, but a clear picture of the TMD and surfactant features that influence the dispersion process is still lacking. In this work, we present a systematic study of the dispersibility of MoS₂, WS₂ and MoSe₂ in aqueous medium using a cationic (cetyltrimethylammonium bromide, CTAB) and an anionic (sodium cholate, SC) dispersant, in a wide concentration range (seven orders of magnitude) and resorting to a carefully controlled sonication-centrifugation procedure. We present detailed, high precision dispersibility curves (concentration of dispersed TMD versus concentration of surfactant used), together with zeta potential and pH measurements, allowing insight into the influence of the type of metal and chalcogen, surfactant charge and surfactant concentration, on the effectiveness of the exfoliation and stabilization. The metal (Mo vs. W) influences the dispersibility at low surfactant concentrations, while the chalcogen (S vs. Se) plays a more significant role as the surfactant concentration is increased, alongside the surfactant charge. Structural characterization by scanning electron microscopy (SEM), Raman spectroscopy and atomic force microscopy (AFM) shows that the methodology applied yields well-exfoliated nanosheets with controlled mean lateral dimension (≈ 100 nm) and thickness (≤5 layers). Finally, the type of ionic surfactant (cationic vs. anionic) and its concentration play a pivotal role in the profile of the dispersibility curves, leading us to propose two types of master curves with distinct regions of phase behavior.

Keywords: 2D layered materials; Dispersion; Liquid-phase exfoliation; Molybdenum diselenide; Molybdenum disulfide; Non-covalent functionalization; Surfactants; Tungsten disulfide.