High-Yield Synthesis of Mesoscopic Conductive and Dispersible

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Article pubs.acs.org/IECR

High-Yield Synthesis of Mesoscopic Conductive and Dispersible Carbon Nanostructures via Ultrasonication of Commercial Precursors Vikram K. Srivastava,*,† Ronald A. Quinlan,‡ Alexander L. Agapov,† Alexander Kisliuk,§ Gajanan S. Bhat,∥ and Jimmy W. Mays*,†,§ †

Department of Chemistry and ∥Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996-1600, United States ‡ Naval Surface Warfare Center Carderock Division, West Bethesda, Maryland 20817-5700, United States § Chemical Sciences Division, Oak Ridge National Laboratory, 1 Bethel Valley, Oak Ridge, Tennessee 37831, United States S Supporting Information *

ABSTRACT: The need to produce large quantities of graphenic materials displaying excellent conductivity, thermal resistance, and tunable properties for industrial applications has spurred interest in new techniques for exfoliating graphite. In this paper, sonication-assisted exfoliation of graphitic precursors in the presence of chloroform is shown to produce chemically and structurally unique exfoliated graphitic materials in high yields. These exfoliated graphites, referred to as mesographite and mesographene, respectively, exhibit unique properties which depend on the number of layers and exfoliation conditions. Structural characterization of mesographene reveals the presence of nanoscale two-dimensional graphene layers, and threedimensional carbon nanostructures sandwiched between layers, similar to those found in ball-milled and intercalated graphites. The conductivities of mesographite and mesographene are 2700 and 2000 S/m, respectively, indicating high conductivity despite flake damage. Optical absorption measurements of mesographite sonicated in various solvents showed significant changes in dispersion characteristics, and also indicated significant changes to mesoscopic colloidal behavior. A mechanism for functionalization and formation of capped carbon nanostructures is proposed by integrating the chemical and structural characterization in relation to the various carbon structures observed by electron microscopy. Composites based on common polymers were prepared by solution processing, and changes in thermal properties indicate improved dispersion of mesographite in polar polymers. sions.20,21 Graphite intercalation compounds (GIC), which have been shown to generate high yields of functionalized graphenes containing only edge defects, can be tailored for excellent dispersibility in various solvents and surfactant/water solutions; however, these treatments also require harsh reagents and extensive processing (thermal or chemical) to generate preferred functionality.19,22 Mild sonication of bromine-GIC (prepared using commercial precursors and a simple dipcoating method) in water indicates that bromine concentration affects interactions between sheets leading to potential tailoring of graphites and graphenes by size, layers, and chemical composition.23 Hernandez et al. demonstrated that by employing sonication the exfoliation of graphite in organic solvents yielded few-layered and monolayered graphenes, displaying only minor structural defects and excellent electrical properties. This group also elucidated the experimental parameters governing solvent exfoliation.24 Modification of this exfoliation procedure has further expanded our understanding of exfoliation of graphite in various liquid media.25−29 Aqueous polymer solutions are capable of graphite exfoliation, producing water-dispersible graphenes.26 Graphite exfoliated by

1. INTRODUCTION Due to the exceptional electronic and thermal properties of graphene, the ability to produce it in large quantities has become a major goal of nanotechnology,1,2 with a potentially huge impact across multiple scientific disciplines, including conductive inks for flexible displays, graphene-based nanomaterials for clean energy applications including photocatalysis,3,4 polymer-based nanocomposites for cars, aircraft, and protective garments, and medical devices and systems. Production of graphene from precursor materials has commonly been performed by two routes: micromechanical exfoliation and chemical exfoliation.2,5−17 The production of graphene from micromechanical exfoliation, the most utilized method, has limitations such as low yield and production of less monolayered graphene and more thicker flakes.2 Exploration of different exfoliation techniques to improve graphene production has broadened the scope of graphene-related research.10,18,19 Chemical exfoliation is an effective process for obtaining monolayer graphene through oxidative reactions to form graphene oxide, which is subsequently reduced.10−17 The chemical polarity of graphite oxide allows for layers of graphene oxide to be peeled off using solvent for exfoliation. This approach yields pristine graphene, but with a yield of only ∼1 wt %.12 Dispersibility of reduced graphene oxide in a range of solvents has been thoroughly explored, giving rise to fundamental information regarding these colloidal suspen© XXXX American Chemical Society

Received: April 23, 2014 Revised: May 15, 2014 Accepted: May 20, 2014

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dx.doi.org/10.1021/ie501659n | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

mL) and diluted with 20 mL of fresh solvent. The tubes were then centrifuged at 3000 rpm for 10 min. The obtained supernatant was collected (∼80%) while not disturbing the pellet at the bottom of the tubes, and the obtained solution was filtered using a 0.22 μm PTFE pore size filter. The obtained products, referred to as nanographite (from precursor graphite) and nanographene (from 20−30 layer precursor graphene), were vacuum-dried at 40 °C for 14 h. The masses of the products were then measured to determine percentage yield, and the products were subsequently characterized. 2.3. Preparation of Polymer Composites. Polymer composites were synthesized by solution processing. The obtained mesographite was dispersed in ∼10 mL of chloroform though momentary sonication to produce dilute dispersions (