Synthesis of silica Janus nanoparticles with metal semishells

Abstract

Dielectric colloids with asymmetric metal shells (Janus particles) have attracted much attention over the years. Their unique properties enable the application in versatile areas. One of the potential application fields is as an adhesion promoter between a dielectric surface and a metal; where one side of the colloid can chemically interact with a polymer substrate, while the other side contains a metal that can directly catalyze the electroless deposition process. In this work, challenges in the fabrication of asymmetric silica colloids with different metallic semishell coatings were investigated; and for the first time, a facile route was found towards the synthesis of large quantities of silica Janus particles with tunable metal shells and thicknesses. First, monodisperse silica particles in sizes ranging from 60–1900 nm in diameter were synthesized as starting materials. Since they were stored as a dry powder, different ultrasonic devices and sonication times were used to deagglomerate the colloids into their initial particle sizes with low polydispersity in a stable solution. Then, the influence of different reaction conditions was tested for binding a monolayer of 3-aminopropyltriethoxysilane (APTES) molecules on the silica colloids in ethanol. The surface functionalization can be enhanced by the presence of a catalyst or an increase in reaction time, or temperature because the reaction is in a thermodynamic equilibrium in protic solvents. An environmental and industrial friendly approach was introduced to synthesize silica particles with one monolayer of APTES. The number of amino groups on the surface was quantified by converting the accessible amino moieties into UV-Vis detectable species with a ninhydrin assay for all colloid sizes, and the C/H/N elemental analysis was used for smaller diameters. Subsequently, silica Janus particles were successfully synthesized with high mass yields (50―70%) when the surfactants didodecyldimethylammonium bromide (DDAB) or cetyltrimethylammonium bromide (CTAB) were present during the preparation of the particle-wax colloidosomes. Even a small concentration of the surfactant was sufficient to improve the mass yield. The radius of gyration of the surfactants was used to calculate the area that the molecules occupy on the silica surface. It was possible to manufacture single colloidosomes with closed-packed monolayers of silica particles on the wax surface. The silica colloids, embedded in wax, were grafted with a monolayer of APTES in the presence of triethylamine (TEA) after reacting for 1 h at 0 °C. Two characterization methods were successfully applied to compare the ratio of grafted and non-grafted area of the silica spheres, the so-called Janus balance. Metal layers were regio-selectively deposited onto the APTES spheres, and the corresponding shell coverage degrees were calculated from the corresponding SEM images. Additionally, the amine density at the surface was quantified with the ninhydrin assay. Both methods demonstrate for both surfactants that the Janus balance on a particle cannot be predicted by varying the amount or concentration of surfactant. In the next step, a method was successfully introduced to coat the APTES-functionalized spheres with Pd seeds with a density of ~30 Pd atoms/nm2, calculated from the ICP-OES results, that catalyzes the electroless deposition of metals onto the activated sphere. Janus particles yielded a surface density of ~20 Pd atoms/nm2, which corresponds to a ~2/3 coverage of the silica colloid, even small amounts of Pd was used. The distribution of Pd clusters was heterogeneous throughout the different batches, which might be caused by the state of the amino groups at the surface of Janus particles or the chain length of the Pd(II) oligomers before the reduction step. The same batch of Pd-seeded precursor particles was utilized for the fabrication of silica-core-metal-nanoshell composites for Ni, Cu, Au, Ag, and Pd using electrolytes reported in the literature, as well as commercially available ones. The layer morphology corresponds directly to the Pd seeds density and distribution on the silica surface, regardless of the deposited metal composition. The metal deposition results showed that in the presence of a complexing agent or a stabilizer, the formation of metal nuclei as side reaction was suppressed. The outcome from the preparation of metal fullshells was used to introduce a general procedure for growing metal semishells with tunable thicknesses for five different metals and electrolytic baths. Different characterization methods were used to prove the shell types and thicknesses, such as inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements, diameter differences measured in SEM; and for selected samples, a cross-section of the colloid using a focused ion beam (FIB).