Controlled assembly of graphene sheets and carbon nanospheres for optimum electrical conductivity in nanostructured coatings

Layer-by-layer assembly (LbL) is emerging as a key nano manufacturing technique that is finding a broad range of applications. It is a versatile, simple and easy to use technique, allowing the realization of novel nanometer-scale multi-layered materials and structures that can be made to have highly desirable properties, including chemical, mechanical, electrical, magnetic, thermal, and optical. The LbL technique allows creation of nanometer scale multilayered films or coatings on arbitrarily shaped objects, by layerby- layer adsorption of alternately charged polyions contained in suspension. Any watersoluble or dispersible elements that have either charges or hydrogen-bonding capability can be considered and built into the resulting films. The substrate can be metal, plastic, ceramic, or other type of material, and the LbL-assembled dispersed phase can consist of polymers, nanoparticles, proteins, or other substances. Despite such long-standing use, LbL assembly of carbon nanoparticles for conductive coating applications has rarely been explored. Carbon nanoparticles of various forms (fullerenes, graphene sheets and platelets, nanotubes and multi-wall nanotubes, amorphous spheres, chains, and fibers) are attracting growing interest in the development of next generation sustainable energy production methods based on the intrinsic electronic properties of each carbon form.;In this work, three-dimensional nanostructure evolution and adsorption kinetics during carbon nanoparticle LbL assembly are investigated for thin film coating applications that require high through-plane electrical conductivity. Two types of nanoparticles are evaluated: 5-10 nm thick stacks of graphene sheets 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 according to the Gouy Chapman Theory. Electrostatic interactions are quantified with electrophoretic mobility zeta potential (zeta) measurements. Suspension pH is used to control the nanoparticle surface charge density through dissociation of hydrolyzed surface groups while the addition of alcohol is used to enhance electrostatic interactions by altering the dielectric constant of the medium. The impact of non-electrostatic interactions is assesed by minimizing zeta.;Alcohol and pH are found to have opposing effects with respect to the packing density and through-plane conductivity of the structures formed for both types of nanoparticles. Such behavior is ascribed to steric effects associated with the heterogeneous dispersion of weakly acidic functional groups on the hydrolyzed carbon nanoparticle surface. Complete dissociation of these groups in the absence of alcohol yields densely packed structures with as much as 40 percent reduction in through-plane conductivity relative to coatings with a greater degree of carbon nanoparticle aggregation and more porous structures. The more densely packed structures involve fewer direct interactions and instead more carbon interactions with the electrically insulating polyelectrolyte binder, yielding the more rapid substrate surface saturation and the higher contact resistance observed. This work was able to establish the LbL process conditions under which carbon coatings sufficiently conductive for a broad range of applications could be produced: from electrostatic discharge (ESD) applications to more stringent fuel cell biplar plate coatings where high through plane conductivity is required.;Carbon nanoplatelet adsorption kinetics was modeled using the Michaelis-Menten (M-M) mechanism, where long-range interaction of the surface with the nanoparticles was hypothesized to yield a reversibly bound species that, with sufficiently strong substrate-adsorbate attraction could become irreversibly bound and thereby incorporated into the LbL assembled structure. Linear irreversible carbon nanoplatelet uptake versus time was observed when electrostatic attraction between the substrate and adsorbate was minimal, consistent with the M-M prediction when irreversible adsorption is rate-limiting due to generally weak substrate-adsorbate interactions. By contrast, non-linear irreversible adsorption kinetics was observed in all cases of enhanced substrate-adsorption electrostatic attraction, whether accomplished through alteration of pH or alcohol content, as is also predicted by M-M model when irreversible adsorption is not rate limiting. Intermediate M-M kinetic behavior was observed during carbon nanosphere adsorption, with neither reversible nor irreversible adsorption dominating.;All carbon nanoparticle coatings produced in this thesis were found to be sufficiently durable for application onto bipolar plates used in proton exchange membrane (PEM) fuel cells.