Formulation of Nickel Oxide–Graphene Composite Ink and the Fabrication of Thin-Film Electrodes Using Direct Ink Writing

Nickel oxide (NiO) is a p-type material with a wide bandgap of 3.6 to 4 eV, and graphene is a zero-bandgap material. When the two materials are combined to form a composite, their structural, optical, and electrical properties are enhanced. This work presents the formulation of nickel oxide (NiO)–graphene composite ink (with ethylene glycol as a solvent) for the fabrication of thin-film electrodes using a direct ink writing technique. NiO is used in nanopowder form, which is prepared via a wet-chemical approach. NiO ink is prepared in three different concentrations of 40 mg/ml (N1), 60 mg/ml (N2), and 70 mg/ml (N3). Graphene ink (G) is also prepared in concentrations of 0.5 mg/ml (G1), 1 mg/ml (G2), and 2 mg/ml (G3). Among the pure NiO inks, it was observed that low resistance and uniform printing were obtained for N2. Therefore, N2 is mixed with different concentrations of graphene to form the composite ink, which is further printed on the substrates. The rheological properties of all the formulated inks are measured. An extrusion-based direct-write system is employed for printing the inks on a glass substrate. The printed samples are characterized before and after annealing at 250°C for 2 h. The resistance of the composite ink (N2G1) is decreased by 53% in comparison with the pure NiO ink (N2). As the conductivity of the composite ink thin film is in the range of MΩ, they could find application in the field of gas sensing and thermistors.