GAO ET AL. VOL. 5 ’ NO. 3 ’ 2134 –2141 ’ 2011 2134 www.acsnano.org February 22, 2011 C 2011 American Chemical Society The Effect of Interlayer Adhesion on the Mechanical Behaviors of Macroscopic Graphene Oxide Papers Yun Gao, †,‡ Lu-Qi Liu, †, * Sheng-Zhen Zu, † Ke Peng, † Ding Zhou, † Bao-Hang Han, †, * and Zhong Zhang †,§, * † National Center for Nanoscience and Technology, China, Beijing 100190, P.R. China, ‡ Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, P.R. China, and § Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, P.R. China O wing to their remarkable thermal, mechanical, electrical, and optical properties, individual graphene nanosheets have been envisioned as novel nanoscale building blocks to create macro- scale graphene-based architectures applied in energy storage technology, 1 composites, 2 mechanical actuators, 3 and optoelectronic devices. 4,5 Over the past few years, several approaches have been developed to pro- duce individual graphene nanosheets in- cluding micromechanical exfoliation of graphite, 6 epitaxial growth through chemi- cal vapor deposition, 7 bottom-up organic synthesis, 8 and chemical modification of graphite. 9 The major advantage of the first three methodologies is the high crystal quality of individual graphene sheets yielded, whereas the mass production of graphene remains a big challenge. Given that natural graphite is ubiquitous and in- expensive in large quantities, the chemical exfoliation of graphite turns out to be a facile and versatile option for the produc- tion of individual graphene sheets in large scale. 10 In addition, the solution-phase fea- ture of chemically exfoliated graphene would also favor subsequent processability and wide-scale applicability. 11 Graphene oxide (GO), as a well-known chemically exfoliated derivative, could be easily dispersed into a variety of solvents because of the presence of carboxylic and hydroxyl groups. 12 The planar feature of a GO sheet makes it easy to assemble into paper-like materials through simple filtra- tion or liquid/air interface self-assembly. 13,14 Mechanical tests have indicated that GO papers exhibit superior stiffness and strength, which surpass most of carbon-based paper- like materials such as buckypaper and flex- ible graphite foil. 10 After further thermal annealing, the resulting paper displays a remarkable mechanical improvement to- gether with high electrical and thermal conductivities. 15 It has to be pointed out that the mechanical properties of macro- scale GO papers are still orders of magni- tude lower than those of individual graphene sheets. Additionally, the reported both Young's modulus and ultimate tensile strength of GO papers, respectively, lay in a relatively wide range of 6-42 GPa and 76-293 MPa. 13-16 Such variance not only arises from the “size-dependent effect” of testing specimens, but also comes from the variety of stacked sheets construction man- ner and interactions in GO papers. It is still a big challenge in the material community so far as to how to create such hierarchically structured nanocomposite in which each sublayer contributes a distinct function to yield a mechanically integrated macro- scopic material. Recently, Ruoff and co-workers 17,18 have pointed out that chemically cross-linking between adjacent GO sheets would impose * Address correspondence to zhong.zhang@nanoctr.cn, hanbh@nanoctr.cn, liulq@nanoctr.cn. Received for review December 6, 2010 and accepted February 4, 2011. Published online 10.1021/nn103331x ABSTRACT High mechanical performances of macroscopic graphene oxide (GO) papers are attracting great interest owing to their merits of lightweight and multiple functionalities. However, the loading role of individual nanosheets and its effect on the mechanical properties of the macroscopic GO papers are not yet well understood. Herein, we effectively tailored the interlayer adhesions of the GO papers by introducing small molecules, that is, glutaraldehyde (GA) and water molecules, into the gallery regions. With the help of in situ Raman spectroscopy, we compared the varied load-reinforcing roles of nanosheets, and further predicted the Young's moduli of the GO papers. Systematic mechanical tests have proven that the enhancement of the tensile modulus and strength of the GA-treated GO paper arose from the improved load-bearing capability of the nanosheets. On the basis of Raman and macroscopic mechanical tests, the influences of interlayer adhesions on the fracture mechanisms of the strained GO papers were inferred. KEYWORDS: graphene oxide . mechanical properties . Raman spectroscopy . interlayer adhesion . strain transfer ARTICLE