Time-resolved SAXS characterization of the shell growth of silica-coated magnetite nanocomposites

Abstract

Time-resolved characterization of silica-coated magnetite nanoparticles during synthesis is performed using our self-developed small-angle X-ray scattering (SAXS) instrument. The shell growth (5–20 nm) is determined quantitatively using a core–shell sphere model. SAXS analyses provide reliable information on the shell thickness despite the complex geometry of the synthesized nanocomposites. They are in good agreement with transmission electron microscope (TEM) observations. Firstly, time-resolved SAXS analyses are used to study the influence of the precursor concentration (tetraethyl orthosilicate) on growth kinetics. Time evolution of the shell thickness can be described by diffusion-limited shell growth, obeying kinetics of first order in the precursor concentration. Furthermore, SAXS measurements provide information on the standard deviation of the shell thickness as a function of the coating time. Its decrease observed with increasing coating time is explained by a self-sharpening mechanism and/or a morphology evolution to more isometric shapes. Additionally, the influence of ammonia is studied. By increasing its concentration, the growth rate is affected. However, the final shell thickness and the standard deviation do not change significantly. For low ammonia concentration, by contrast, the SAXS and TEM observations reveal superimposed silica gelation. In addition, coating reaction was conducted at elevated temperature (40 °C). SAXS patterns measured as a function of the coating time and TEM micrographs reveal simultaneous production of classic Stober particles under these conditions. Hence, SAXS is found to have a big potential for on-line monitoring of the shell properties.