2, The George Washington University, Washington, District of Columbia, United States
Proposed applications of drop-on-demand inkjet printing extend beyond the printing of text and graphics to include rapid manufacturing of thin electrical devices. The most common material for printing electronics is silver nanoparticles. However, silver is expensive, and metal nanoparticle films require post-processing to remove polymeric stabilizers and recover high conductance. The post-processing step restricts substrate choice and slows throughput. These limitations are shared by other traditional electronic materials such as copper, gold, and aluminum. Oxidation of nanoparticles is another challenge that severely reduces or prevents electrical conductance, especially for copper and aluminum, and necessitates specialized processing and handling.
Carbon nanomaterials are a promising alternative to metallic nanoparticles because of their high electrical conductivity and chemical stability. Current carbon nanomaterial inks suffer from low concentrations because it is difficult to disperse carbon nanomaterials in common, environmentally-safe solvents. Low particle concentration prevents sufficient percolation in printed materials, so electrical conductivity is low. Electrostatic stabilization improves nanoparticle stability – and thus increases concentration – in liquid media. The surface charges of nanoparticles in a liquid form an electrical double layer with dissolved ions. When two particles approach each other, their double layers overlap resulting in a repulsive electrostatic force. The zeta potential characterizes the magnitude of the electric potential between the particle double layer and the bulk liquid and is measurable with dynamic light scattering techniques.
Here we investigate the electrostatic stability of a novel carbon nanomaterial, hollow carbon nanospheres1 (HCNS), in water and ethylene glycol. The HCNS consist of concentric graphene spheres and are made from charred cellulose, the byproduct of a biofuel production process. We measure the zeta potential and particle size distribution as a function of pH to identify solvent parameters that maximize the electrostatic stability of the carbon nanomaterial. We observe an increased sensitivity of zeta potential to changes in pH for HCNS in water over those in ethylene glycol. The maximum observed zeta potential of HCNS in ethylene glycol is 50% less than those in water at the same pH. The results suggest an aqueous based HCNS ink is more stable than one based in ethylene glycol. By performing pH-titrations with HCNS, we identify the presence and concentration of ionizing surface groups as a function of pH and deduce their relative contributions to the electrical double layer. We also compare the maximum concentration of HCNS in water and ethylene glycol to commercial metal nanoparticle inks.
1M. J. Wagner, J. Cox, T. McKinnon, and K. Gneshin, “Hollow carbon nanosphere based secondary cell electrodes.” U.S. Patent 8262942, issued September 11, 2012.