(Nanowerk Spotlight) The successful implementation of graphene-based devices invariably requires the precise patterning of graphene sheets at both the micrometer and nanometer scale. It appears that 3D-printing techniques are an attractive fabrication route towards three-dimensional graphene structures. In a previous Nanowerk Spotlight we reported on the first 3D printed nanostructures made entirely of graphene.
There are also different methods to build 3D graphene monoliths – for example freeze casting or emulsion templating, etc. – but they are limited to building simple shapes, for example cylinders or cubes.
Using a different approach, researchers have now used flakes of chemically modified graphene – namely graphene oxide GO and its reduced form rGO – together with very small amounts of a responsive polymer (a polymer that changes behavior and conformation when a 'chemical switch' is activated), to formulate water based ink or pastes.
"Our formulations have the flow and physical properties we need for the filament deposition process required in 3D printing: They need to flow through very small nozzles and set immediately after passing through it, retaining the shape and holding the layers on top," Dr. Esther García-Tuñon, a Research Associate at the Centre for Advanced Structural Ceramics at Imperial College London (ICL), tells Nanowerk. "We use this two-dimensional material as building block to create macroscopic 3D structures and a technique called direct ink writing (DIW) also known as direct write assembly (DWA), or Robocasting."
García-Tuñon is first author of a paper in the January 21, 2015 online edition of Advanced Materials ("Printing in Three Dimensions with Graphene") where a team from ICL, the University of Warwick, the University of Bath, and the Universidad de Santiago de Compostela, describe their technique.
This technique is based on the continuous deposition of a filament following a computer design. The 3D structures are built layer by layer from bottom to top.
a) Piled up filaments and b) woodpile GO freeze-dried structures (printed through 500 and 150 µm nozzles respectively) after drying. The woodpile structure exhibits good bonding between layers and good definition. c,d) Fracture of a junction in a woodpile structure printed through 500 µm nozzle showing the inner microstructure of the printed lines. The lines have densities ranging from 25 to 65 mg cm-3 depending on ink formulation (before thermal reduction to rGO). The ice crystals formed during freeze-drying template the formation of a porous architecture inside the printed lines at the microscopic level resulting in highly porous structures with smooth surfaces. The inset in (d) shows an EDS analysis of the freeze-dried structure before reduction. Quantitative EDS analysis before reduction indicate that the dried structures are composed mainly by carbon, oxygen, and small amounts of Na (62% C, 35% O, and 3%Na). Peaks not labeled correspond to the conductive Au coating used for scanning electron microscopy. The sodium impurities come from the NaOH used to regulate the pH during the formulation of the inks. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
"Our inks allow printing through nozzles as thin as 100 µm and their rheology could also be tailored for other processing technologies such as extrusion, gel, or tape casting," García-Tuñon points out. "Our goal was to print graphene structures (not composites) using small amounts of additives and water-based systems. In this way it could be easily scaled up in a manufacturing process."
Graphene is very hydrophobic so it is not possible to formulate a water-based ink directly. The researchers therefore used chemically modified graphene – also known as graphene oxide (GO) – instead. GO can be processed in water to build the desired architectures.
Once the structure is made, it is thermally treated in a special atmosphere to recover the properties of graphene.
The mechanical stability of the printed parts may allow the use of additional treatments (e.g., chemical or electrochemical reduction) to further manipulate the properties without compromising the structure.
The team is now working on developing new formulations as well as the development of specific applications for example in flexible electronics and oil adsorption.
"I think there are still many challenges to overcome in both Additive Manufacturing and graphene technologies," notes García-Tuñon. "The buzzword 3D printing is now everywhere; we can find many examples of commercially available 3D printers to make your own Hello Kitty, iPhone cases, and all sort of plastic models. But there is still a long way to go from here to the use of 3D printing for a wide variety of materials in multicomponent and practical devices."