A simple synthesis strategy could enable carbon nanomaterials to retain their unique properties in three-dimensions, say researchers. The team from Case Western Reserve University, Georgia Institute of Technology, University of North Texas, Air Force Research Laboratory in Dayton, Wenzhou Medical University, and Beijing Institute of Nanoenergy and Nanosystems has devised a novel one-step process to create seamless graphene—carbon nanotube (CNT) threedimensional nanostructures [Xue et al., Sci. Adv. (2015), 10.1126/sciadv.1400198].
One-dimensional carbon nanotubes and two-dimensional graphene boast impressive thermal, electrical, and mechanical properties in-plane but poor properties in three dimensions because of weak van der Waals interactions between layers. Now Liming Dai and colleagues have created hollow fibers consisting of radially aligned CNTs (RACNTs) attached to cylindrical graphene layers (Fig. 1) with a seamless junction between the two materials.
‘‘In our one-step process, the interface is made with carbon—to—carbon bonding so it looks as if it’s one single graphene sheet,’’ explains Dai. ‘‘That makes it an excellent thermal and electrical conductor in all planes.’’
The large surface area (up to nearly 527 m2/g) and minimal interfacial electrical and thermal resistance of the three-dimensional graphene—RACNT material is ideal for energy storage in devices like batteries or supercapacitors and energy conversion in solar cells. The fibers are so flexible that they can even be woven into fabrics for novel wearable power sources.
Demonstration supercapacitors based on the graphene— RACNT fibers have energy storage capacities up to four times greater than other fiber-based devices, report the researchers. And as a counter electrode in dye-sensitized solar cells, the graphene—RACNT fibers can boost power conversion efficiencies to 6.8%.
‘‘This opens a new path for the design and growth of various three-dimensional graphene—CNT architectures with novel properties unobtainable with one-dimensional CNTs or two-dimensional graphene, while maintaining the excellent properties of their building blocks,’’ says Dai.
A simple aluminum wire is the starting point for the team’s one-step process. The wire is first etched to create radially aligned nanoscale holes along the length and circumference. Chemical vapor deposition is then used to deposit graphene on top of the wire, with CNTs growing in the holes without the need for any metal catalyst particles.
‘‘The elimination of nanoparticle catalysts avoids detrimental effects on the interfacial mechanical and transport properties,’’ explains Dai.
The aluminum template is then etched away to leave behind a three-dimensional structure, which can be tailored by varying the length of the template wire, its diameter, and the density of holes.
A number of nanocarbon-based materials have been already developed for applications in energy storage and conversion, points out Philippe Poulin of the Centre de Recherche Paul Pascal in Bordeaux, France. But these, by and large, have been in the form of planar electrodes.
‘‘Xue et al. have achieved an exciting advance by making materials in the form of flexible and robust fibers. These materials could be useful for future smart textiles that can store or produce electrical energy,’’ Poulin told Nano Today.
This article originally appeared in Nano Today (2015), doi:10.1016/j.nantod.2015.10.005
Figure 1 Schematic of the synthesis and microstructure of graphene—RACNT fibers. (A) Aluminum wire. (B) Surface anodized aluminum (AAO) wire. (C) Three-dimensional graphene—RACNT structure on AAO wire. (D) Schematic of graphene—RACNT structure. (E—G) Top view scanning electron micrographs of graphene—RACNT fiber at different magnifications. (I—K) Scanning electron micrographs of cross-section of graphene—RACNT structure. (H and L) Atomic force microscopy of graphene—RACNT fiber. (M—P) Scanning electron micrograph (M) and corresponding EDX elemental mapping of aluminum (N), oxygen (O), and carbon (P) of graphene—RACNT fiber