ECCM15 - 15 TH EUROPEAN CONFERENCE ON COMPOSITE MATERIALS, Venice, Italy, 24-28 June 2012 1 EFFECTS OF REDUCTION AND SIZE OF GRAPHENE ON MECHANICAL AND ELECTRICAL PROPERTIES OF GRAPHENE OXIDE PAPERS Xiuyi Lin, Qingbin Zheng, Nariman Yousefi, Kan Kan Yeung, Xi Shen, Jang-Kyo Kim * Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong *e-mail address of the corresponding author: mejkkim@ust.hk Keywords: Graphene oxide, graphene oxide paper, mechanical property. Abstract The effects of graphene oxide (GO) size and reduction method on mechanical property and electrical conductivity of GO papers are studied. GO sheets are prepared using the modified Hummers method and GO papers are fabricated through vacuum filtration of GO aqueous dispersions. To study the size effect, the as-prepared GO dispersion is sorted into four different groups with uniform sizes after repeated centrifugation and collection of supernatant. It is found that the Young’s modulus, tensile strength and electrical conductivity increase consistently with increasing the GO size. Three methods, including hydrazine reduction, hydrogen iodide reduction and thermal treatment, are employed to reduce the GO sheets. The highest electrical conductivity of 139,000 S/m is obtained after thermal reduction. The mechanisms responsible for these observations are discussed based on the elemental, structural and morphological analyses of GO and reduced GO sheets. 1 Introduction Graphene oxide (GO) consists of a single or a few layer graphene sheets with oxygenated functional groups attached on their basal planes and edges. GO paper is a free-standing paper- like material that is fabricated by stacking GO sheets via flow-directed assembly of aqueous GO dispersion. This new material outperforms many other paper-like materials in mechanical properties [1]. The tensile modulus and strength of GO paper are found to be comparable to those of flexible graphite foils and pristine carbon nanotube bucky papers prepared by filtration [2]. The combination of exceptional mechanical properties [3], thermal stability [4], high electrical conductivity [5] and biocompatibility [6] make GO papers a promising candidate for many applications, ranging from free-standing, flexible electrodes for Li ion batteries [7] to biomedical implants [8]. The lateral dimensions of precursor GO sheets are considered playing an important role in controlling properties and applications of GO papers. For example, large GO sheets are ideally suited in a number of applications, e.g. forming three-dimensional graphene-based networks [9], developing an aligned structure in polymer-based composites [10] and producing conductive thin films for optoelectronic devices [11]. Larger GO sheets can give rise to a higher electrical conductivity due to lower inter-sheet contact resistance than in small GO [12]. The thermal conductivity of GO papers was also a strong function of GO sheet size