Graphene Oxide Papers Simultaneously Doped with Mg 2+ and Cl - for Exceptional Mechanical, Electrical, and Dielectric Properties Xiuyi Lin, Xi Shen, Xinying Sun, Xu Liu, Ying Wu, Zhenyu Wang, and Jang-Kyo Kim* Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ABSTRACT: This paper reports simultaneous modification of graphene oxide (GO) papers by functionalization with MgCl 2 . The Mg 2+ ions enhance both the interlayer cross-links and lateral bridging between the edges of adjacent GO sheets by forming Mg-O bonds. The improved load transfer between the GO sheets gives rise to a maximum of 200 and 400% increases in Young’s modulus and tensile strength of GO papers. The intercalation of chlorine between the GO layers alters the properties of GO papers in two ways by forming ionic Cl - and covalent C-Cl bonds. The p-doping effect arising from Cl contributes to large enhancements in electrical conductivities of GO papers, with a remarkable 2500-fold surge in the through-thickness direction. The layered structure and the anisotropic electrical conductivities of reduced GO papers naturally create numerous nanocapacitors that lead to charge accumulation based on the Maxwell-Wagner (MW) polarization. The combined effect of much promoted dipolar polarizations due to Mg-O, C-Cl, and Cl - species results in an exceptionally high dielectric constant greater than 60 000 and a dielectric loss of 3 at 1 kHz by doping with 2 mM MgCl 2 . The excellent mechanical and electrical properties along with unique dielectric performance shown by the modified GO and rGO papers open new avenues for niche applications, such as electromagnetic interference shielding materials. KEYWORDS: graphene oxide paper, Mg and Cl doping, electrical conductivity, mechanical properties, dielectric constant ■ INTRODUCTION Two-dimensional graphene and graphene-based materials have attracted tremendous interests from various research commun- ities and much attention has been drawn to explore their exceptional characteristics for real-world applications. 1,2 Among several well-known techniques to synthesize graphene and its derivatives, the chemical method based on the earlier work 3 is the most popular and practical way to obtain graphene oxide (GO) in an aqueous dispersion. GO consists of mono- to several-layer graphene sheets with oxygenated functional groups on their basal plane and edges, and can be reassembled into thin films 4,5 or paperlike materials 6-9 in a freestanding form. GO papers have received growing interests owing to their unique structure, properties and potential multifunctional applications. Graphene-based papers with a modulus up to 35 GPa and tensile strength higher than 120 MPa, 6 bring about better bendability than CNT bucky paper and graphite foil. They could absorb up to 0.92 MJ/kg of ballistic energy, 10 times the amount of energy steel can, making them superb body armor. 7 After effective reduction, the electrical con- ductivity of reduced GO papers could reach 139 000 S/cm 8 and free-standing flexible graphene papers have been successfully used as current collector and binder free anodes for lithium ion batteries with excellent capacities above 330 mAh g -1 after 100 cycles. 9 These graphene-based papers have already been extensively employed in energy storage, strain sensors, sealants, actuators, biocompatible substrates, etc. 10 Together with a high dielectric constant of greater than 15 000, the highly conductive graphene papers can also find many applications in flexible electronics, electric power systems, and electromagnetic interference shielding. 11 Although an individual graphene sheet has an extremely high modulus and strength, the corresponding properties of GO papers are far below those of the monolayer pristine graphene, Received: November 26, 2015 Accepted: January 8, 2016 Research Article www.acsami.org © XXXX American Chemical Society A DOI: 10.1021/acsami.5b11486 ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX