Highly controllable and green reduction of graphene oxide to flexible graphene film with high strength Wubo Wan a , Zongbin Zhao a, *, Han Hu a , Yury Gogotsi a,b , Jieshan Qiu a, * a Carbon Research Laboratory, Liaoning Key Lab for Energy Materials and Chemical Engineering, State Key Lab of Fine Chemicals, Dalian University of Technology, Dalian 116024, China b Department of Materials Science and Engineering, and A.J. Drexel Nanotechnology Institute, Drexel University, Philadelphia, PA 19104, USA 1. Introduction Graphene has attracted enormous attention in recent years due to its extraordinary physical and chemical properties [1–3]. However, mass production of high quality graphene sheets at low cost is needed for commercial applications [4]. Recently, several methods have been developed to synthesize graphene sheets [5], such as micromechanical exfoliation of graphite [1], chemical vapor deposition [6] and epitaxial growth on silicon carbide [7,8], but chemical reduction of graphene oxide (GO) is regarded as the most promising approach for large volume synthesis. However, harmful or hazardous chemicals (such as hydrazine and sodium borohydride) are usually employed in the chemical route [9–12]. Therefore, it is important to develop environmentally friendly methods for producing graphene nanosheets in large scale [13–21]. Sodium citrate, a harmless natural food additive, is widely used in Turkevich method for the preparation of monodisperse gold nanoparticles under mild conditions [22]. In the present work, we report on the large scale production of graphene using a citrate reduction of GO. This eco-friendly method has the advantages of low cost and good scalability. Moreover, the as-made chemically converted graphene (CCG) can self-assembly, resulting in high- quality graphene films that are strong, flexible and electrically conductive. Because of this, the graphene films are of great potential in many fields such as in supercapacitors and batteries. 2. Experimental 2.1. Preparation of GO Graphite oxide was synthesized by the modified Hummers method [23,24]. The exfoliation of graphite oxide to GO was accomplished by ultrasonication of the yellow brown neutral graphite oxide solution for 30 min (see Supplementary Material for details). 2.2. Reduction of GO by sodium citrate In a typical procedure for the reduction of graphene oxide to graphene, 150 mg of GO was added into 250 ml of deionized (DI) water and then treated by ultrasonication for 30 min. After that, 5 g sodium citrate was added into the GO suspension (corresponding to the concentration of 20 mg/ml), and the reaction was carried out at 95 8C. Finally, the resulting black mixture was cooled to room temperature and the CCG suspension was washed until neutral. In order to investigate the reduction process from GO to CCG, the reaction was conducted under a series of conditions by varying the temperature from 35 8C to 95 8C and the concentration of the reducing agent from 0 to 20 mg/ml. Materials Research Bulletin 48 (2013) 4797–4803 A R T I C L E I N F O Article history: Received 13 January 2013 Received in revised form 9 June 2013 Accepted 18 August 2013 Available online 29 August 2013 Keywords: A. Nanostructures B. Chemical synthesis C. X-ray diffraction D. Mechanical properties A B S T R A C T Graphene film with high strength was fabricated by the assembly of graphene sheets derived from graphene oxide (GO) in an effective and environmentally friendly approach. Highly controllable reduction of GO to chemical converted graphene (CCG) was achieved with sodium citrate as a facile reductant, in which the reduction process was monitored by XRD analysis and UV–vis absorption spectra. Self-assembly of the as-made CCG sheets results in a flexible CCG film. This method may open an avenue to the easy and scalable preparation of graphene film with high strength which has promising potentials in many fields where strong, flexible and electrically conductive films are highly demanded. ß 2013 Elsevier Ltd. All rights reserved. * Corresponding authors. Tel.: +86 411 84986072; fax: +86 411 84986080. E-mail addresses: zbzhao@dlut.edu.cn (Z. Zhao), jqiu@dlut.edu.cn (J. Qiu). Contents lists available at ScienceDirect Materials Research Bulletin jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/mat res b u 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.08.031