www.afm-journal.de FULL PAPER © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.MaterialsViews.com wileyonlinelibrary.com Seung Yol Jeong, Sung Hun Kim, Joong Tark Han, Hee Jin Jeong, Soo Yeon Jeong, and Geon-Woong Lee* 1. Introduction Graphene has received much interest due to its superior mechanical and electrical properties, including a high Young’s modulus, high specific surface area, high electrical conduc- tivity, and high carrier mobility. [1–4] Single-layer graphene is a promising candidate for the production of electronic devices, composites, and energy storage applications. Sev- eral methods for producing single-layer graphene, including mechanical cleavage, epitaxial growth, chemical vapor deposi- tion (CVD), and chemically modified graphene (CMG), have been described. [5–8] The CVD and CMG methods provide the most practical approaches for adaptation to large-scale manufacturing processes. Here, we focused on the use of CMG for solution-based electronics printing and the mass production of high concentrated and conductive single-layer graphene. The pro- duction of CMG involves several essential steps for the chemical exfoliation and sub- sequent reduction of graphite oxide. [9] Col- loidal suspensions of high-quality CMG, which contain few oxygen functional groups and defects, have been unobtain- able due to the hydrophobic nature of graphene. The synthesis of graphite oxide was described by Brodie in 1859 as a starting material for graphene. [10] The synthetic methods typically used today are those of Brodie, Saudenmaier, Hummers, and Offman and involve extensive oxida- tion by a strong acid to increase the inter- layer distance in the graphite. Hummers’ method is the most common, and is the fastest and most effective way to form an aqueous dispersion with a large interlayer distance typical of highly oxidized graphite oxide. [11] Compared to the Hummers method, Brodie’s method yields lower contamination and higher quality graphite oxide, although the interlayer distance is small. [12,13] To improve the exfoliation of graphite oxide prepared by Brodie’s method and the dispersion GO, the pH should be controlled by addition of NaOH or KOH. We used a modified Brodie method accordingly to produce high-quality RGO. However, reduction of GO to RGO tends to result in agglomeration in aqueous solutions due to hydrophobic interactions among the RGO sheets. [14] A stable dispersion of RGO is particularly important for preserving the unique properties of the nanostructures. Surfactants, such as polymers, have been introduced to reduce agglomeration; [15] however, the presence of polymers in RGO can alter the charac- teristics of RGO that are most desirable for applications. Using a modified Hummers method, Li et al. prepared an aqueous dispersion of RGO without a polymeric surfactant by control- ling the pH using ammonium ions and dialysis. [16] Park et al. reported the preparation of a colloidal suspension containing hydrazine-reduced potassium-modified RGO by addition of aqueous KOH. These studies suggested that the introduction Highly Concentrated and Conductive Reduced Graphene Oxide Nanosheets by Monovalent Cation– π Interaction: Toward Printed Electronics A novel route to preparing highly concentrated and conductive reduced graphene oxide (RGO) in various solvents by monovalent cation– π interac- tion. Previously, the hydrophobic properties of high-quality RGO containing few defects and oxygen moieties have precluded the formation of stable dispersion in various solvents. Cation– π interaction between monovalent cations, such as Na + or K + , and six-membered sp 2 carbons on graphene were achieved by simple aging process of graphene oxide (GO) nanosheets dispersed in alkali solvent. The noncovalent binding forces introduced by the cation– π interactions were evident from the chemical shift of the sp 2 peak in the solid 13 C NMR spectra. Raman spectra and the I- V characteristics dem- onstrated the interactions in terms of the presence of n-type doping effect due to the adsorption of cations with high electron mobility (39 cm 2 /Vs). The RGO film prepared without a post-annealing process displayed superior elec- trical conductivity of 97,500 S/m at a thickness of 1.7 μm. Moreover, mass production of GO paste with a concentration as high as 20 g/L was achieved by accelerating the cation– π interactions with densification process. This strategy can facilitate the development of large scalable production methods for preparing printed electronics made from high-quality RGO nanosheets. DOI: 10.1002/adfm.201200242 Dr. S. Y. Jeong, S. H. Kim, Dr. J. T. Han, Dr. H. J. Jeong, S. Y. Jeong, Dr. G.-W. Lee Graphene Hybrid World Class Laboratory Nano Carbon Materials Research Group Korea Electrotechnology Research Institute (KERI) Changwon, 641-120, Korea E-mail: gwleephd@keri.re.kr Adv. Funct. Mater. 2012, DOI: 10.1002/adfm.201200242