Targeting Chemical Morphology of Graphene Oxide for Self-Assembly and Subsequent Templating of Nanoparticles: A Composite Approaching Capacitance Limits in Graphene

The chemical morphology of graphene oxide sheets (GO) is characterized by the presence of functional carboxylic groups (at the edges) and epoxy and hydroxyl groups (at basal plane). Using divalent calcium ions, this morphology is used to bind the sheets together into an exfoliated sheet assembly or compact multilayers by controlling the pH. The assembly provides a 3-D template where the adjacent GO sheets mechanically constraint the synthesis of nanomaterials. Using this template, nanoparticles are directly synthesized on the sheets with high density and uniform shape and size, without a capping agent. Both noble metal Au and Ag nanoparticles and inorganic CaCO3 nanoparticles are synthesized. The formation of nanoparticles leads to local straining of the sheets, and the location of the nanoparticles coincides with the formation of radial wrinkles on the sheets. Further electrochemical capacitors made with GO-CaCO3 composite shows capacitance values of 240 F/g, approaching the theoretical limits of 550 F/g, as the CaCO3 nanoparticles act as spacers between the sheets preserving their surface area.