(382e) Thin-Film Nanostructure Development During the Aggregation of Graphene Nanoplatelets and Amorphous Carbon Nanospheres Onto Charged Surfaces

The assembly of carbon nanoparticles of various forms (fullerenes, graphene sheets and platelets, single- and multi-walled nanotubes, amorphous spheres, chains, and fibers) into thin films is of growing interest in the development of a new generation of highly efficient devices such as photovoltaics and fuel cells for sustainable energy production. However, the tendency of carbon materials to form poorly defined aggregates with binders in make-up solutions has typically yielded sub-optimal performance of these devices due to enhanced scattering of free electrons, reduced electron mobility, and low electronic conductivity. In this work, thin film nanostructure evolution and aggregation dynamics during carbon nanoparticle self-assembly onto charged surfaces are examined for thin film coating applications that require high through-plane electrical conductivity. Two types of nanoparticles are evaluated: 5-10 nm thick stacks of asymmetric graphene sheets (graphene nanoplatelets) and 20 nm diameter amorphous carbon spheres. Electrostatic interactions between the carbon nanoparticles and a cationic polyacrylamide binder are systematically altered by varying the carbon nanoparticle suspending media composition and quantified with electrophoretic mobility zeta potential measurements using dynamic light scattering. SEM and HAADF-STEM are used to visualize nanoparticle aggregation behavior and thin film nanostructure development. The nanoparticle surface charge density is systematically varied through dissociation of hydrolyzed surface groups with the suspension pH while the addition of alcohol is used to enhance electrostatic interactions by altering the dielectric constant of the medium. Alcohol and pH are found to have opposing effects with respect to nanoparticle aggregation into islands and the resultant through-plane conductivity of thin films that are formed on surfaces. Such behavior is ascribed to steric effects associated with the heterogeneous dispersion of weakly acidic functional groups on the hydrolyzed carbon nanoparticle surface; whose dissociation regulates electrostatic interaction with the cationic polyelectrolyte binder and other nanoparticles during assembly. Complete dissociation of these groups maximizes nanoparticle adsorption to oppositely charged surfaces and minimizes aggregation with other nanoparticles. The resultant uniformly layered thin film structures are found to have 40% lower through-plane conductivity relative to the most highly aggregated structures of similar thickness, independent of nanoparticle shape.