Inkjet printing of graphene
The inkjet printing of graphene is a cost-effective, and versatile deposition technique for both transparent and non-transparent conductive films. Printing graphene on paper is aimed at low-end, high-volume applications, i.e., in electromagnetic shielding, photovoltaics or, e.g., as a replacement for the metal in antennas of radio-frequency identification devices, thereby improving their recyclability and biocompatibility. Here, we present a comparison of two graphene inks, one prepared by the solubilization of expanded graphite in the presence of a surface active polymer, and the other by covalent graphene functionalization followed by redispersion in a solvent but without a surfactant. The non-oxidative functionalization of graphite in the form of a donor-type graphite intercalation compound was carried out by a Birch-type alkylation, where graphene can be viewed as a macrocarbanion. To increase the amount of functionalization we employed a graphite precursor with a high edge to bulk carbon ratio, thus, allowing us to achieve up to six weight percent of functional groups. The functionalized graphene can be readily dispersed at concentrations of up to 3 mg ml−1 in non-toxic organic solvents, and is colloidally stable for more than 2 months. The two inks are readily inkjet printable with good to satisfactory spreading. Analysis of the sheet resistance of the deposited films demonstrated that the inks based on expanded graphite outperform the functionalized graphene inks, possibly due to the significantly larger graphene sheet size in the former, which minimizes the number of sheet-to-sheet contacts along the conductive path. We found that the sheet resistance of printed large-area films decreased with an increase of the number of printed layers. Conductivity levels reached approximately 1–2 kΩ □−1 for 15 printing passes, which roughly equals a film thickness of 800 nm for expanded graphite based inks, and 2 MΩ □−1 for 15 printing passes of functionalized graphene, having a film thickness of 900 nm. Our results show that ink preparation and inkjet printing of graphene-based inks is simple and efficient, and therefore has a high potential to compete with other conductive ink formulations for large-area printing of conductive films.