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.

Competing effects of silanol surface concentration and solvent dielectric constant on electrostatic layer-by-layer assembly of silica nanoparticles on gold

“…The impact of such alteration on the electrostatic force is not in accordance with the Scheutjens-Fleer model of polymer adsorption [6]. In a more recent investigation [4], a reduction in silica adsorption qualitatively similar to that of [2] was observed when the pH of a strong cationic polymer suspension used as the complementary binder was increased, yielding an increase in the absolute value of the negative surface charge of preadsorbed silica nanoparticles. In contrast to this behavior, when the absolute value of the (negative) nanoparticle f potential was increased in the same study by reducing the dielectric constant of the suspending media through the addition of ethanol, the amount of silica deposited increased.…”

“…The two types of silica nanoparticles and the cationic binder employed in this study were obtained from commercial sources and have previously been characterized extensively [4]. Silica nanoparticles used for polishing applications were obtained from Electron Microscopy Supply (EMS) and have a diameter of 65 nm, as measured using photon correlation spectroscopy (PCS).…”

“…Silica nanoparticles with a diameter of 19 nm were obtained from Nano-X GmbH (NanoX) and have the trade name X-Tec 3408. The polydispersity for both of these types of silica was found to be quite high, with an average value of 0.12 for the EMS and 0.29 for the NanoX [4]. Weakly acidic silanol groups are responsible for the anionic surface charge on both silica nanoparticles [8].…”

“…Electrostatic layer-by-layer assembly of silica nanoparticles is of growing interest in applications ranging from antifog and antireflectance coatings [1][2][3] to hydrophilic bipolar plate coatings in proton-exchange membrane (PEM) fuel cells [4] to the development of new flocculants [5]. Nanostructure development during assembly is a key product parameter in such applications.…”

“…Nanostructure development during assembly is a key product parameter in such applications. For example, power loss in PEM fuel cells is found to increase with the bilayer (silica nanoparticle plus cationic polymer) thickness [4]. A detailed understanding of the role of intermolecular interactions in the assembly process is the key to optimizing coating nanostructure for a given application.…”