University of Groningen A Simple Road for the Transformation of Few-Layer Graphene into MWNTs

We report the direct formation of multiwalled carbon nanotubes (MWNT) by ultrasonication of graphite in dimethylformamide (DMF) upon addition of ferrocene aldehyde (Fc-CHO). The tubular structures appear exclusively at the edges of graphene layers and contain Fe clusters. Fc in conjunction with benzyl aldehyde, or other Fc derivatives, does not induce formation of NT. Higher amounts of Fc-CHO added to the dispersion do not increase significantly MWNT formation. Increasing the temperature reduces the amount of formation of MWNTs and shows the key role of ultrasound-induced cavitation energy. It is concluded that Fc-CHO first reduces the concentration of radical reactive species that slice graphene into small moieties, localizes itself at the edges of graphene, templates the rolling up of a sheet to form a nanoscroll, where it remains trapped, and finally accepts and donates unpaired electron to the graphene edges and converts the less stable scroll into a MWNT. This new methodology matches the long held notion that CNTs are rolled up graphene layers. The proposed mechanism is general and will lead to control the production of carbon nanostructures by simple ultrasonication treatments. ■ INTRODUCTION Carbon is a most versatile element that occurs in allotropic forms as diverse as diamond and graphite and in the more recently discovered nanostructures of fullerenes, nanotubes (CNTs), and graphene. The production of graphene by micromechanical cleavage triggered enormous experimental activity. Many studies demonstrated that graphene monolayers possess novel structural, electrical, and mechanical properties. Additionally, graphene can be thought as a 2D building block for carbon nanostructures of other dimensionalities. It can be wrapped into 0D buckyballs, rolled into 1D nanotubes, or stacked into 3D graphite. Recently, in situ TEM experiments demonstrated the direct transformation of flat graphene sheets into fullerene cages where etching of the edge carbon atoms promotes folding into fullerenes. CNTs are often described as rolled-up graphene layers. Matching this concept to experiments where the layers fold into CNTs is still a great challenge. To date large-scale mass CNT production has only been achieved by stochastic synthetic processes, such as arc discharge, laser ablation, and chemical vapor deposition (CVD), which require postsynthetic separation and purification treatments. During the past few years, ultrasonication has become an extremely powerful tool in the synthesis, modification, and manipulation of carbon nanomaterials. Under appropriate conditions, ultrasounds can functionalize CNTs, open their caps, or even fracture them completely. In addition to surface modifications, CNTs can be prepared directly from organic solvents with the assistance of ultrasounds. Graphite ultrasonication produces exfoliation in many solvents, if the free energy of mixing is negative and the solvent is able to stabilize colloidal graphene. It is accepted that ultrasounds break the graphitic basal structure and produce graphitic carbon fragments of variable sizes, which are later intercalated by solvent molecules. To complicate matters, ultrasounds generate cavities whose implosion releases sufficient energy to form high-energy intermediates and free radicals that can drive chemical reactions. Chemical attack reduces the size of the graphene sheets and is therefore detrimental to the physical properties that are usually sought after. Graphene dispersions produced by exfoliation of graphite in organic solvents, such as N-methyl-2-pyrrolidone (NMP) and N,N-dimethylformamide (DMF), first reached concentrations up to 0.01 mg/mL and 1 wt % monolayer. Increasing sonication time, the concentration increased up to 1.2 mg/mL and 4 wt% monolayer. Received: April 1, 2012 Published: May 7, 2012 Article