Silica Nanoparticle Layer-by-Layer Assembly on Gold

Abstract

Layer-by-layer (LBL) assembly of silica nanoparticles is investigated as a means of controlling the surface wetting properties of gold electroplated onto 316 L stainless-steel substrates while maintaining a low electrical surface contact resistance. The strong polyelectrolyte acrylamide/beta-methacryl-oxyethyl-trimethyl-ammonium copolymer is used as the cationic binder. The impact of silica nanoparticle zeta (zeta) potential for a range of -37.1 to 5.9 mV in the thickness, wettability, and contact resistance of the final LBL-assembled coatings is presented. The zeta potential is varied by altering both the pH and alcohol (ethanol) content of the silica suspensions and polymer suspension, consistent with the predictions of the Debye-Huckel equation. Nanoparticle adsorption is found to occur rapidly, with surface coverage equilibration obtained after only 1 min and uptake that is nearly linear with respect to the number of bilayers deposited. An increase in the absolute value of the (negative) zeta potential in the silica suspension is found to increase the bilayer thickness to an average value as high as 82% of the individual nanoparticle diameter for the smaller nanoparticles investigated, suggesting that nearly complete surface coverage may be achieved after the application of only a single nanoparticle-polymer bilayer (a coating thickness as low as 15.6 nm) and that nanoparticle adsorption is enhanced by electrostatic attraction between substrate and adsorbate. Counterintuitively, a more porous bilayer structure is observed if the zeta potential of the previously deposited nanoparticles is increased while the substrate is immersed in the cationic copolymer suspension, suggesting that copolymer adsorption in inhibited by substrate-solvent interactions. Wetting measurements demonstrate that silica LBL assembly results in a substantial reduction in contact angle from 84 degrees on the bare substrate surface to as low as 15 degrees after the application of a single bilayer and 7 degrees after the application of eight bilayers. A monotonic increase in coating contact resistance is observed with an increase in the thickness with a characteristic volumetric electrical through-plane resistivity of as low as 1.63 kOmega.cm obtained from contact resistance measurement.