Controlled Size Reduction of Liquid Exfoliated Graphene Micro-Sheets via Tip Sonication

Chiara Di Berardino,Péter Bélteky,Fabian Schmitz,Francesco Lamberti,Teresa Gatti,Enzo Menna,Ákos Kukovecz

doi:10.3390/cryst10111049

Chiara Di Berardino, Péter Bélteky + Show 5 more

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https://doi.org/10.3390/cryst10111049

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Abstract

Liquid exfoliation of three-dimensional bulk solids with an inherent layered structure is an effective and scalable method to produce stable re-aggregation colloidal inks of 2D materials that are suitable for solution processing. Shear mixing is a relatively gentle technique that allows exfoliation while preserving the native lateral size of the 3D precursors, while tip sonication often leads to extensive structural damage, producing 2D sheets where many edge defects are introduced. We present a mixed approach to obtain liquid dispersions of few-layer graphene flakes, wherein the average lateral size of the colloids can be tuned in a controlled way. This strategy relies on the application of defined tip sonication steps on graphene inks previously prepared through the use of a shear mixer, thus starting with already-exfoliated micro-sheets with a limited amount of edge defects. Our approach could represent a valuable method to prepare 2D material inks with variable size distributions, as differences in this parameter could have a significant impact on the electronic behavior of the final material and thus on its field of application.

Highlights

The production of 2D materials through scalable methods that can preserve their quality and ensure their industrial exploitation in a wide variety of devices [1,2,3,4] and applications [5,6,7,8] is a major concern the applied research of today [9]
We report on the use of a two-step liquid-phase exfoliation (LPE) method that allows us to control the average size of few-layer graphene sheets dispersed in CHP, with significant relevance to the future production of size-defined graphene inks for solution processing
This colloidal sample is named EXG 3000 after the purification protocol employed. This consists of three subsequent centrifugation steps at increasing centrifugation speeds, the last being carried out at 3000 rpm (1170× g) and allowing for the isolation of the most exfoliated fraction within the mixture that underwent LPE. To this graphene micro-sheet ink, tip sonication processes of different durations and power were applied and the resulting products were characterized by Raman spectroscopy, dynamic light scattering (DLS), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA)

Summary

Introduction

The production of 2D materials through scalable methods that can preserve their quality and ensure their industrial exploitation in a wide variety of devices [1,2,3,4] and applications [5,6,7,8] is a major concern the applied research of today [9]. In order to find a cheaper and less hazardous process for large-scale graphene production, liquid-phase exfoliation (LPE) methods have been developed in the last few years [12,13]. These techniques provide a system with the required energy to overcome the van der Waals forces between layers in 3D crystals without increasing the in-plane defect degree. The LPE process generally involves threes steps, namely the dispersion of the bulk material in a given solvent or water/surfactant solution, the exfoliation itself and the purification. The last step (normally carried out through liquid cascade centrifugation) is Crystals 2020, 10, 1049; doi:10.3390/cryst10111049 www.mdpi.com/journal/crystals

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