Elsevier

Surfaces and Interfaces

Volume 20, September 2020, 100548
Surfaces and Interfaces

Exfoliation synthesis of graphene and optimization with alkali halide salts

https://doi.org/10.1016/j.surfin.2020.100548Get rights and content

Abstract

Exfoliation synthesis of graphene requires a lesser amount of wasteful chemicals and due to the hydrophobic nature, a naturally separated graphene layer is obtained with the method. In this study, three combinations of simple mechanical processes were utilized for the exfoliation synthesis of graphene sheets. The synthesized graphene sheets were characterized with TEM, SAED, SEM and EDS techniques. Based on the characterization results, the method that produced the best quality of graphene sheets was selected for further quality optimization study with alkali halide salts. Three different salts were chosen for the investigation. In the literature, there were several articles reporting improvement in flake size and the quality of the synthesized graphene sheets on account of using NaCl, sulfur, etc. Intercalation of salt particles between the layers, acting as tiny wedges to facilitate fluent exfoliation, was reported as the possible mechanism. Contrary to the popular notion of the quality improvement of the synthesized graphene sheets with such intercalation attempts, our characterization results conclusively demonstrate that the quality does not improve.

Introduction

Graphite consists of scores of single atom thick, strongly knitted, two-dimensional graphene sheets that are stacked together with weaker Van der Waal interaction. In mechanical synthesis approach, stacked graphene layers are carefully separated using appropriate mechanical forces. The first successful isolation of a single-layer graphene was also achieved using mechanical exfoliation of graphite in 2004 by Andre Giem and Konstantin Novoselov [1,2]. Thereafter, synthesis of graphene has been reported using several other methods such as chemical vapor deposition (CVD) [3,4,5], thermal decomposition of SiC [6], chemical reduction of graphene-oxide [7,8], etc. Exotic properties of graphene, for instance, prominent mechanical strength [9,10], copious electrical conductivity [11,12], high optical transparency [13,14,15], ultrahigh electron mobility [16] and superior thermal conductivity [17,18], make it a center of attention of researchers from diverse disciplines. In multiple biomedical sensors, graphene [19] and reduced graphene oxide [20,21] based composites have been reported, including in a sensor for sensitive detection of toxic antibiotic in milk [22].

An economical and simpler synthesis method is desirable for mass-scale applications, preferably a synthesis method without involving chemicals and chemical reactions. Liquid-based exfoliation synthesis of graphene requires among the least amount of wasteful chemicals and due to the hydrophobicity of graphene, a self separating graphene layer floats on the liquid surface at the end of the synthesis process. In the present work, three combinations of common mechanical processes (sonication, planetary ball-milling and centrifugation) were utilized to synthesize graphene sheets. Thus synthesized three different samples of graphene were characterized using transmission electron microscopy (TEM), selected area electron diffraction (SAED), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The analysis of the characterization results conclusively established that three times centrifugation of 6 hours ball-milled graphite powder was the best method among the investigated methods.

There were several articles that reported domain size and quality improvement of the synthesized graphene sheets with NaCl [23] and sulfur [24], due to possible intercalation of the particles acting as minuscule wedges between the layers and facilitating a smoother exfoliation. In order to confirm the above findings, an optimization study was conducted with three alkali halide salts (NaCl, KCl and LiI) at two concentrations (20% and 40%). The characterization results establish that the flake size and quality substantially suffer due to the presence of the salt particles during the synthesis.

Section snippets

Materials and methods

Graphite powder (purity 99.8%) used in the synthesis was purchased from Sigma Aldrich. The base material for each sample was 5 gm graphite powder. Sodium chloride (NaCl, purity 99.5%) and Potassium chloride (KCl, purity 99.0%) were purchased from Thomas Baker. Lithium iodide (LiI, purity 99.0%) was purchased from Otto Chemie. Ethanol and isopropyl alcohol (IPA) used for cleaning beakers, spatula, etc., were purchased from local vendors. Fresh double distilled water (DI) was obtained as needed

Exfoliation without salts

In this part, characterization results of synthesized graphene sheets (without any salt) are discussed. As presented in Table 1, three combinations of common mechanical processes were utilized for the synthesis of graphene sheets.

Sample 1

Ultra-sonication of graphite powder

This is one of the simplest exfoliation synthesis methods to produce graphene flakes. It is crucial to point out that no chemical was used during ultra-sonication. Graphite powder (5 gm) underwent only a three-step ultra-sonication

Conclusion

Three combinations of mechanical processes were employed for exfoliation synthesis of graphene sheets. The characterization results were analyzed and compared to find out the best method among the three utilized methods. Based on the results, it was established that the three times centrifugation after a 6 hrs ball-milling of graphite powder (Sample-3) was the best method. Further optimization of the synthesis method was attempted by adding different alkali halide salts before the milling of

Author Statement

Md. Furqan Alam: was responsible for following protocols of the synthesis, maintain logbook of samples, collect in-house measurement data and schedule appointment for SEM, EDS, TEM and SAED measurements. Wrote the first draft of the manuscript.

Mohd. Shoeb Khan: participated in discussions.

Imran Uddin: Helped in sample preparation for SEM & TEM measurements.

Shahper Nazeer Khan: participated in discussions.

Irfan Ahmad: Conceptulized the investigation, prepared detailed protocols for the

Declaration of Competing Interests

None

Acknowledgement

The authors gratefully acknowledge the support of AMU through INC departmental/laboratory funds for providing TEM grids, ITO substrates and chemicals used in the work. The authors acknowledge the help of staff members of USIF-AMU for SEM, and TEM measurements. MFA would like to acknowledge TEQUIP-III scholarship to GATE qualified M Tech student from MHRD, Government of India 2.

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