Water-based and inkjet printable inks made by electrochemically exfoliated graphene

Inkjet printable graphene inks are very attractive for applications in flexible and foldable electronics, such as wearable electronics and the Internet of Things. However, the ink preparation is still very time consuming as high concentrations can be achieved only with prolonged sonication (>24 h)or with expensive setups. Here we demonstrate a water-based inkjet printable ink made from electrochemically exfoliated graphene. A printable and stable (>1 month)ink with concentration of ∼2.25 mg mL⁻¹ was formulated in less than 5 h, using two successive steps: first exfoliation and dispersion of large graphene flakes (>5 μm)followed by 1 h tip-sonication to reduce the flake size below 1 μm, as required for the material to be ejected by the nozzle. The formulated ink contains more than 75% single- and few-layers (i.e. less than 2 nm in thickness)graphene flakes with an average lateral size of 740 nm. Thermal annealing allows to achieve high C/O ratio (>10), which translates into one of the highest electrical conductivity (≈3.91 × 10⁴ S m⁻¹)reported so far for solution-processed graphene, without the use of any harsh chemical processing.