  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
hours) 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 hrs, using two successive steps: first exfoliation and dispersion
of large graphene flakes (> 5 um) followed by 1 hour tip-sonication to reduce
the flake size below 1 um, 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 x 104 S m-1)
reported so far for solution-processed graphene, without the use of any harsh
chemical processing.
