Nanoscale PAPER Cite this: Nanoscale, 2017, 9, 4024 Received 15th January 2017, Accepted 18th February 2017 DOI: 10.1039/c7nr00341b rsc.li/nanoscale Microwave reduction of graphene oxide rationalized by reactive molecular dynamics Vitaly V. Chaban a and Oleg V. Prezhdo* b Obtaining graphene (GRA) in industrial quantities is among the most urgent goals in today’s nanotechno- logy. Elegant methods involve the oxidation of graphite with its subsequent solvent-assisted exfoliation. The reduction of graphene oxide (GO) is challenging leading to a highly-disordered oxygen-rich material. A particularly successful microwave-induced reduction of GO was reported recently (Science, 2016, 353, 1413–1416). We mimic the experiment by reactive molecular dynamics and establish the molecular mechanisms of reduction and their time scales as functions of temperature. We show that the rapid removal of oxygen groups achieved by microwave heating leaves GRA sheets intact. The epoxy groups are most stable within GO. They can rearrange into the carbonyl groups upon quick heating. It is impor- tant to avoid creating holes upon graphite oxidation. They cannot be healed easily and undermine GRA thermal stabilityand electronic properties. The edge oxygen groups cannot be removed by irradiation, but their effect is marginal on the properties of μm GRA sheets. We demonstrate that different oxygen groups are removed from GO at drastically different temperatures. Therefore, it is possible to obtain separate fractions, e.g. carbonyl-, hydroxyl- and carboxyl-free partially reduced GO. Our results guide the improvement of the GO reduction methods and can be tested directly by experiment. 1. Introduction The solvent-assisted exfoliation of graphite is a promising experimental technique for the production of graphene (GRA) in high (industrial) quantities. 1–19 However, only low yields of single-layer GRA, with generally broad thickness distributions of multi-layer GRA, have been observed through exfoliation thus far. Exfoliation is thermodynamically unfavorable and, therefore, requires external investments of energy (sonication, centrifugation, electrochemical exfoliation, etc.). The intercala- tion of electrolytes has been found useful to enhance exfoliation. 6,12,20–25 Surfactants (amphiphilic molecules) are able to change favorably the thermodynamics of exfoliation. 9,26–28 The restacking of GRA after exfoliation is possible and should be efficiently prevented. 29,30 A thoughtful selection of the exfoliation solvent is very important. 4,8,31–35 The treatment of graphite with strong oxidizers (Hummer’s method) produces graphite oxide, also frequently referred to as graphitic oxide and graphitic acid (GA). GA is composed of carbon, oxygen, and hydrogen in variable ratios depending on the oxidation conditions. 36–38 The usually reported carbon to oxygen ratios in GA range from 2.1 to 2.9. 39–41 Efforts to prepare GA with a tailored percentage of oxygen (oxidation degree) are known to match certain application needs. 42 High fractions of oxygen result in a lower-density, ∼1000 kg m -3 , material with large and irregular inter-layer spacing. GA is interesting for a variety of applications. 43–46 GA readily under- goes exfoliation in water and aqueous solutions producing gra- phene oxide 47 (GO) with yields of up to 100%. 10,48,49 However, GO lacks several favorable properties of GRA. 50,51 Removal of the oxygen functional groups is challenging, 52,53 leading to a highly disordered material with drastically inferior properties. It has not been possible yet to obtain good-quality GRA from GO. Therefore, the product of GO reduction is called reduced GO (rGO) to underline its difference from pristine GRA. While GO and rGO can be useful in catalysis, energy storage and other applications, 54–59 their utility in electronics is much less feasible. Sustainable methods enabling the efficient chemical modification of GO and rGO into GRA would be of high importance. Recently, Voiry and co-workers 60 reported an interesting GO reduction method by using 1- to 2-second long microwave pulses. Upon irradiation, large arcing was observed around GO typically lasting 50–100 ms. The authors correlate arcing with a vigorous GO reduction. An estimated short-time growth of the GO temperature is a few thousand degrees celsius. Prior to microwaving, a mild annealing was performed at 300 °C for 1 hour under an argon atmosphere. The authors compared microwaving with and without annealing at 300 °C, and con- a Instituto de Ciência e Tecnologia, Universidade Federal de São Paulo, 12247-014 São José dos Campos, SP, Brazil. E-mail: vvchaban@gmail.com b Department of Chemistry, University of Southern California, Los Angeles, CA 90089, USA. E-mail: prezhdo@usc.edu 4024 | Nanoscale, 2017, 9, 4024–4033 This journal is © The Royal Society of Chemistry 2017 Published on 21 February 2017. Downloaded by University of Southern California on 11/9/2019 9:28:35 PM. View Article Online View Journal | View Issue