Noble-metal-promoted three-dimensional macroassembly of single-layered graphene oxide.

The integration of nanostructured materials into macroscopic devices that can translate phenomena at the nanoscale to the macroscopic level has proved key to paving the way to realizing applications of nanomaterials. To date, although remarkable progress has been made in the self-assembly of building blocks such as nanocrystals, nanotubes, nanowires, and the newly discovered graphenes, very little success has been achieved with three-dimensional (3D) macroscale assemblies. Herein we report the controlled assembly of single-layered graphene oxide (GO) into 3D macrostructures promoted by a noble-metal nanocrystal (Au, Ag, Pd, Ir, Rh, or Pt, etc.). Although the density of such macroassemblies is very low (ca. 0.03 gcm ), they have shown excellent mechanical properties, and have been utilized as fixed-bed catalysts for a Heck reaction resulting in both 100% selectivity and conversion. We expect our endeavor may further the research and practical applications of graphene-based materials. Graphene is a well-defined two-dimensional structure of carbon atoms. It has received a great deal of attention because of its unique electronic, thermal, and mechanical properties. Micromechanical cleavage from highly ordered pyrolytic graphite (HOPG) and the reduction of exfoliated graphite oxide sheets (graphene oxide, GO) are commonly used to produce graphene; the reduction of GO appears to be a viable approach to the large-scale production of graphene. GO is usually prepared by oxidation of graphite powder with strong oxidants such as a mixture of concentrated sulfuric acid and potassium permanganate. The abundant oxygen-containing groups on GO endow it with excellent aqueous dispersion and make it easy to modify. Recently, efforts have focused on the fabrication of graphene films through the self-assembly of the well-dispersed GO or functionalized graphene sheets. Herein, we demonstrate that the 3D assembly of GO into macroscopic porous structures has been achieved successfully with the assistance of noble-metal nanocrystals (Au, Ag, Pd, Ir, Rh, Pt, etc.). The macroscopic size of the obtained samples can be easily controlled by adjusting the volume of the vessel, and the microstructure (the number of the pores and the pore diameter) can be controlled by varying the effective concentration of GO. These results mean that the macroscopic and microscopic structure of the sample can be controlled in one step. It not only provides a simple way to fabricate porous structures from GO, but also shows that the GO can selfassemble into more complicated 3D structures. Furthermore, the combination of noble-metal nanocrystals and the GO single layers may result in some interesting properties. As an example, we show that the Pd-embedded assemblies exhibited excellent catalytic activity and selectivity for the Heck reaction; this indicates that the self-assembled 3D structure is an ideal catalyst for a fixedor fluid-bed reactor. An important property of GO is that it can be readily exfoliated and forms stable colloidal suspensions in water. In our work, a dilute (1 mgmL ) suspension of GO is prepared by using ultrasound methods, and the suspension then undergoes mild centrifugation. The as-prepared colloidal suspensions mainly comprise GO sheets with uniform thickness. Figure 1a shows the atomic force microscopy (AFM) image of a typical GO sheet, and Figure 1b shows the corresponding height profile. It shows that the graphene monolayer is irregular, having a height of about 0.7 nm and an edge that is slightly higher because of the crinkles; this data is consistent with the height of a GO monolayer. There are hardly any GO sheets that are either thicker or thinner than 0.7 nm, indicating the complete exfoliation of GO in the experiment. When the GO suspension containing a noble-metal salt and glucose is hydrothermally treated at high temperature, a 3D structure is obtained. To obtain an unchanged sample, a freeze-drying process is needed. Figure 1c–e shows macroscopic views of the 3D self-assembled samples. The cylindrical morphology of the sample conforms to the morphology of the vessel. It is interesting that the size of the obtained samples can be controlled easily by the volume of the container. When 35 mL of the GO mixture is contained in a vessel having a volume of 50 mL, the diameter of the obtained cylinder is about 1.2 cm and the height is about 2 cm (Figure 1c,d); when 20 mL of GO mixture is placed in a 30 mL vessel, the diameter and height of the sample decrease to about 0.7 cm and 1.2 cm, respectively (Figure 1e). Furthermore, when the effective concentration of the GO suspension is changed, the size of the 3D self-assembled sample is not changed as long as the volume of the GO mixture is maintained. This result indicates that the size of the sample is mainly determined by the volume of the container, and the larger sample can be easily prepared provided the container is large enough. Interestingly, the 3D cylinder exhibits considerable mechanical strength. (Figure 1 f–h). Figure 1g visually shows that the 3D cylinder can support a weight of at least 330 g even though [*] Dr. Z. H. Tang, Dr. S. L. Shen, Prof. J. Zhuang, Prof. X. Wang Department of Chemistry, Tsinghua University Beijing 100084 (P. R. China) Fax: (+86)10-6279-2791 E-mail: wangxun@mail.tsinghua. edu.cn