Reduction of graphene oxide through a green and metal-free approach using formic acid Mousumi Mitra a , Krishanu Chatterjee a , Kajari Kargupta b , Saibal Ganguly c , Dipali Banerjee a, ⁎ a Department of Physics, Bengal Engineering & Science University, Shibpur, Howrah, India b Department of Chemical Engineering, Jadavpur University, Kolkata, India c Chemical Engineering Department, Universiti Teknologi Petronas, Malaysia abstract article info Article history: Received 25 February 2013 Received in revised form 10 May 2013 Accepted 11 May 2013 Available online 23 May 2013 Keywords: Graphene oxide Chemical reduction Formic acid Reduced graphene oxides Electrical conductivity The present work reports on the production of reduced graphene oxide (GO) by the chemical reduction of GO using formic acid. The process involved is simple, environmentally friendly, low cost and metal free. The structural and electrical characterization ascertains that the quality of the material improves with the time of reduction. To compare the effect of reduction time, three samples are prepared for 18, 24 and 30 h respec- tively. The samples produced are characterized to confirm the reduction of GO and formation of reduced GO (FRGO) by high-resolution transmission electron microscopy, field emission scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction pattern, ultraviolet visible spectroscopy and Raman spectroscopy. Among the three samples, FRGO-3 prepared for reduction time of 30 h shows a good crystalline behavior and the highest electrical conductivity (11.859 S/cm) at room temperature. This value is comparable with other reported values. Further, from thermo-gravimetric analysis reasonable thermal stability for FRGO-3 is observed in the temperature range 400–800 °C. Based on the above observations a mechanism of reduction from GO to reduced GO by formic acid (FRGO) is proposed. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Invention of graphene [1] has opened up a new field of research due to its exceptional mechanical, physical and chemical proper- ties [2,3]. Graphene has broadened the applications in several areas such as electronic devices [4], energy storage systems [5–7], com- posite materials [8,9], solar cells [10], drug delivery [11], field effect transistors [12], sensors [13], fuel cells [14], super capacitors [15], transparent electrodes [16] and electromechanical resonators [17]. Till date, several approaches like micromechanical exfoliation of graphite [1], chemical vapor deposition growth [18–20], exfoliation of carbon nanotubes [21,22], and epitaxial graphene growth [23] have been undertaken to prepare graphene. These approaches involve high cost and low producibility. So, the solution-based graphene prepa- ration comes in forefront. Generally, graphene is prepared by oxidation of graphite powder using modified Hummer's method [24] and succes- sive reduction procedure. Several routes have been adopted to prepare graphene by reduction of graphene oxide (GO) via electrochemical reduction [25–27], photo catalytic reduction [28,29], thermal reduc- tion [30,31] and chemical reduction with different reducing agents like hydrazine hydrate [32], NaBH 4 [33], LiAlH 4 [34] some metals like aluminum powder [35], metallic zinc [36], sulphur-containing com- pounds [37], different acids like vitamin-c [38], l-ascorbic acid [39], oxalic acid [40], hydrohalic acid [41], hydriodic acid with acetic acid (HI–AcOH) [42] which are promising. Recently some more new strate- gies emerged for the synthesis of graphene like reduction of exfoliated GO with Fe [43], hydrothermal dehydration for the green reduction of exfoliated GO to graphene [44], graphene sheets using pyrrole [45], solvothermally reduced GO using N-methyl-2-pyrrolidinone (NMP) [46], reduction of aqueous suspension of GO by nascent hydrogen [47], reduction using glycine [48] and dimethyl ketoxime [49]. In this work a simple, less toxic, low-cost and metal-free method of synthesis of reduced GO using formic acid (FRGO) is presented. Formic acid is a reducing organic acid widely used in many redox re- actions. The mechanism of formation of reduced GO from GO has been proposed. The quality of the material with regards to its electri- cal property improves as the time of reduction increases. 2. Experimental 2.1. Materials used Purified graphite powder (99.99%, crystalline, 60 mesh, LOBA Chemie), sodium nitrate (NaNO 3 ), potassium permanganate (KMnO 4 ), sulphuric acid (H 2 SO 4 ), ortho-phosphoric acid (H 3 PO 4 ), hydrogen per- oxide (H 2 O 2 ), calcium chloride (CaCl 2 ), phosphorus pentoxide (P 2 O 5 ), and formic acid (HCOOH) were purchased from MERCK. All the other Diamond & Related Materials 37 (2013) 74–79 ⁎ Corresponding author. Tel.: +91 9830299253; fax: +91 3326684561. E-mail addresses: dipalibanerjeebesu@gmail.com, banerjee_dipali@yahoo.co.in (D. Banerjee). 0925-9635/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.diamond.2013.05.003 Contents lists available at SciVerse ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond