Author's personal copy Flexible conductive graphene paper obtained by direct and gentle annealing of graphene oxide paper Cristina Valle ´s, J. David Nu ´n ˜ ez, Ana M. Benito, Wolfgang K. Maser * Department of Chemical Processes and Nanotechnology, Instituto de Carboquı ´mica ICB-CSIC, C/Miguel Luesma Casta ´ n 4, E-50018 Zaragoza, Spain ARTICLE INFO Article history: Received 4 July 2011 Accepted 21 September 2011 Available online 29 September 2011 ABSTRACT We report the fabrication of flexible conductive graphene paper through a direct and gentle annealing process of graphene oxide paper. Thermal treatments at 700 °C under argon or hydrogen atmosphere directly applied to parent graphene oxide paper lead to a significant removal of disruptive oxygen-containing functional groups, and to a considerable recovery of the sp 2 network structure. Detailed comparison of chemical and combined chemical– thermal treatments by scanning electronic microscopy (SEM), Raman, X-photoelectron spectroscopy (XPS) and conductivity measurements underline the high efficiency of the direct annealing process. The resulting highly reduced graphene oxide paper exhibits elec- trical conductivities as high as 8100 S/m representing an increase of five orders of magni- tude with respect to the parent graphene oxide paper, which significantly outperforms the results of chemical treatments. Moreover, our direct and gentle thermal reduction allows maintaining the structural integrity and mechanical flexibility of the parent graph- ene oxide paper thus overcoming problems of brittleness typically encountered in anneal- ing processes. Our approach sets the base for an easy, cost-effective and environmentally friendly fabrication route for flexible conducting graphene paper of great application poten- tial as flexible electrodes in various fields of technology. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction There is currently a great demand for the development of new inexpensive, flexible, robust, light-weight and environmen- tally friendly paper-like materials. Here especially carbon- based paper materials have raised great interest for various types of applications such as electrochemical energy storage devices [1–3], catalyst supports and fuel cells [4–6], sensors and actuators [7–10], chemical filters and membranes [11,12], and structural composites [13,14], to name only a few. A promising building block of intriguing actual scientific and technological interest is graphene, best described as a two-dimensional single-atom thick sheet of sp 2 hybridized carbon atoms arranged in a honeycomb lattice with outstand- ing electronic, mechanical, thermal and chemical properties [15,16]. Recently individual water soluble graphene oxide (GO) sheets [17] – heavily oxygenated graphene sheets bearing epoxy and hydroxyl functional groups on their basal planes, in addition to carbonyl and carboxyl groups located at the sheet edges – were assembled by a flow-directed vacuum fil- tration process into an interlocking-tile arrangement yielding free-standing and highly flexible GO paper [18]. While the presence of oxygen moieties provides GO sheets their hydro- philic character and imparts versatile high volume processing possibilities from stable aqueous suspensions into GO paper and other macroscopic assemblies [19], they also heavily dis- rupt the conjugated sp 2 network of the basal plane of the indi- vidual graphene sheets and drastically degrade their electric 0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.09.042 * Corresponding author: Fax: +34 976 73 3318. E-mail address: wmaser@icb.csic.es (W.K. Maser). CARBON 50 (2012) 835 – 844 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon