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The Noble-Prize-winning discovery of graphene has created an entirely new branch of materials science and technology. Being a single-atom-thick sheet of carbon atoms packed in two-dimensional (2D) honeycomb lattices, graphene possesses a large surface area and high electrical/thermal conductivity as well as excellent mechanical properties. These interesting properties make graphene attractive for a variety of potential applications, including electronic devices, solar cells, supercapacitors, batteries, fuel cells, sensors, and actuators. However, graphene sheets without functionalization are insoluble and infusible, and the poor processability has limited their large-scale practical applications. Recent effort has led to solution-processable graphene oxides (GOs) from exfoliation of graphite by acid oxidation. Subsequent reduction of GOs yields reduced graphene oxide (rGO) or graphene nanosheets (GNs). The availability of solution-processable GOs has not only allowed for the formation of GN films through various solution processing methods but also facilitated functionalization of GNs. Having a large number of oxygen-containing functional groups (e.g., carboxyl, hydroxyl, epoxy groups), GOs could be used as strong oxidizing reagents. While the oxygencontaining groups of GOs have often been removed via chemical reduction to produce GNs or used as functional sites for chemically bonding other moieties, there is little discussion on potential use of GOs as oxidizing reagents (catalysts). Along with the recent intensive effort in developing metal-free catalysts based on carbon nanomaterials (e.g., N-doped carbon nanotubes, N-doped graphene), we found that GOs could act as strong oxidizing reagents to effectively oxidize Fe into Fe3O4 nanoparticles, which simultaneously deposited on the self-reduced GO surface. As we shall see later, the resultant Fe3O4-nanoparticle-decorated reduced graphene oxide (Fe3O4/rGO) shows interesting magnetic and electrochemical behaviors useful for potential energy storage, catalytic, and even biomedical applications (e.g., supercapacitors, magnetic bioimaging and targeted drug delivery). As indicated by the present work, GO can be used to oxidize many metal and even non-metal ions. Therefore, the methodology developed in this study could be regarded as a facile, but effective and versatile, approach toward the fabrication of reduced graphene sheets decorated with many other metal oxide nanoparticles of practical significance. In a typical experiment, GO was prepared by acid oxidation of graphite powder according to the modified Hummers method (ESIw). Instead of reducing the resultant GO with those widely-used highly toxic/explosive reduction reagents, such as hydrazine and NaBH4, 15 we used the GO as an efficient oxidizing reagent to oxidize Fe from FeCl2 or FeSO4 to form Fe3O4/rGO via the spontaneous in situ deposition of Fe3O4 nanoparticles onto the self-reduced GO surface in this study. As shown in Scheme 1a, the Fe3O4/rGO can be prepared via a redox reaction between GO and Fe. The redox reaction was evident by a color change from yellow (Scheme 1b), characteristic of GO in an aqueous solution of NH4OH (pH = 9), to dark black (Scheme 1c, left panel) upon addition of a predetermined amount of FeCl2 (weight ratio of FeCl2 4H2O to GO = 10 : 1). These Fe3O4-nanoparticle-decorated rGO showed strong attraction towards an external magnet, leading to an almost full separation of the Fe3O4/rGO out of the solution (Scheme 1c, right panel). This clearly indicates that Fe3O4 nanoparticles have imparted useful magnetic properties to the rGO. Fig. 1a andb showatomic forcemicroscopic (AFM,Agilent 5500 AFM) images of theGOandFe3O4/rGO.As can be seen in Fig. 1a, a Institute of Advanced Materials for Nano-Bio Applications, School of Ophthalmology & Optometry, Wenzhou Medical College, 270 Xueyuan Xi Road, Wenzhou, Zhejiang325027, China. E-mail: [email protected] Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106, USA. E-mail: [email protected] c Polymer Science Institute, University of Akron, Akron, Ohio 44325, USA. E-mail: [email protected] w These authors contributed equally. ChemComm Dynamic Article Links