Graphene Based Inks for Printed Electronics

Abstract: The outstanding properties of graphene make it attractive ink filler for conductive inks which plays an important role in printed electronics. This thesis focuses on the ink formulation based on graphene and graphene oxide (GO).Liquid phase exfoliation of graphite is employed to prepare graphene dispersions, i.e., shear- and electrochemical exfoliation. High concentration graphene dispersions with small size, few-layer graphene platelets are obtained by both methods. With the addition of ethyl cellulose stabilizer, shear-exfoliated graphene platelets in NMP were successfully inkjet printed on different substrates. The printed graphene film with electrical conductivity of ~3^104 S/m was obtained after annealing at 350 °C for one hour. Alternatively, the electrochemically exfoliated graphene nano-platelets were collected and redispersed in DMF to form inks. The printed film of conductivity ~2.5^103 S/m was obtained after annealing at 300 °C for one hour.Water based GO/Ag hybrid inks were developed for screen printing. When high concentration GO aqueous dispersion was mixed with reactive silver ink, the viscosity of the mixture increased instantly to above 1000 cP as a result of reactions between oxygen functional groups (OFGs) on GO sheets and ingredients in the reactive silver ink. When the screen printed lines with different GO:Ag ratios were annealed in air, the conductivity of the resultant reduced graphene oxide/silver nanoparticles (RGO/AgNPX) composites decreased as silver content increased. As oxygen enriched compounds in RGO/AgNPX composites were detected, we proposed that AgOx compounds were generated on the AgNPs surface, which raised the contact resistance between AgNPs and RGO flakes. To solve this problem, the printed patterns were instead annealed in reducing gas (Ar/H2 5%). The electrical conductivity ~2.0^104 S/m was then achieved.Furthermore, the reduction of GO using ammonium formate as reducing reagent was investigated. When applying a hydrothermal method, ammonium formate shows excellent reduction ability, surpassing the widely used reducing agent, L-ascorbic acid, under same condition. Elemental analysis shows the C/O ratio of RGO as high as ~11 and most OFGs were removed in the reduction process. Meanwhile, incorporated nitrogen atoms introduced active sites in resultant RGO, making it a promising electrocatalyst for oxygen reduction reaction.