Investigation of the liquid exfoliation of 2D materials using the nanoindentation technique

  • Zainab Al Kharusi

Student thesis: Phd

Abstract

The selection of suitable solvents for the exfoliation of 2D materials from their bulk form is typically based on semi-empirical correlations based upon the surface energy terms 1,2. In this study, we propose a rapid screening and selection method for solvents using nanoindentation, specifically targeting the exfoliation of graphene and other 2D materials. We measure the first recorded dislocation in the load to displacement curve (pop-in) during loading as an indicator of solvent effectiveness. Our analysis of nanoindentation load-displacement curves on highly oriented polycrystalline graphite (HOPG) immersed in pure solvents reveals that the minimum average first pop-in load is influenced by solvent composition. For example, nanoindentation tests conducted on HOPG immersed in N-Methyl-2-pyrrolidone (NMP), a commonly used exfoliation solvent, demonstrate significantly lower mean load for the first pop-in load (4.58 ± 0.81 mN) compared to dry HOPG (8.59 ± 0.31 mN). While the first pop-in load for pure water, IPA, and ethanol (8.74 ± 0.28 mN, 9.07 ± 0.49 mN, and 8.76 ± 0.51 mN, respectively) were comparable to that of dry HOPG (8.85 ± 0.44 mN), the influence of these solvents on HOPG exfoliation remains uncertain. Subsequent experiments investigated the impact of solvent mixtures on the mechanical properties and exfoliation efficacy of 2D materials. Introducing specific proportions of ethanol or isopropyl alcohol (IPA) into water resulted in a noticeable reduction in the average first pop-in load for HOPG and MoS2, respectively. Optimal solvent ratios were determined, with HOPG exhibiting improved properties at volume ratios of 70% ethanol (4.92 ± 0.36 mN) to water and 20% IPA (6.21 ± 0.22 mN) to water mixture. Moreover, nanoindentation experiments on MoS2 in IPA/water mixtures validated this correlation, highlighting the optimal composition at around 40% IPA (6.06 ± 0.51 mN). Ultrasonic bath sonication experiments support these findings, emphasizing solvent composition's impact on material dispersion. Raman microscopy analysis of the nanoindentation sites for different solvents reveals significant changes in the HOPG bands, indicating substantial structural modifications in the indented material. The localised contact of the high-load tip during indentation causes damage to the HOPG, leading to deformation in the region beneath the indentation. We then adapted the indentation studies to understand the electrochemical exfoliation of graphene from graphite where ions are intercalated into the galleries within the graphite. A specially designed electrochemical cell was used in the nanoindentator to evaluate the exfoliation efficiency of three different aqueous inorganic salt aqueous electrolytes; ammonium sulphate ((NH4)2SO4), sodium sulphate (Na2SO4), and potassium sulphate (K2SO4). The analysis of the load-displacement curves obtained revealed a linear correlation between the ionic size of the cations and the minimum average first pop-in load. The results demonstrate the potential of the nanoindentation technique combined with electrochemical exfoliation for the systematic screening and selection of ions to produce high-quality graphene and other 2D materials. This approach offers valuable insights into the fundamental mechanisms underlying the exfoliation process and paves the way for further advancements in the field of 2D material synthesis and applications.
Date of Award1 Aug 2024
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorBrian Derby (Supervisor) & Ian Kinloch (Supervisor)

Keywords

  • exfoliation of 2D materials
  • Graphene
  • Nanomaterials
  • nanoindeters

Cite this

'