Black is the new orange: inline synthesis of silica-coated iron oxide nanoparticles produced via gas-phase in a matrix burner

Abstract

Superparamagnetic iron oxide nanoparticles (IONPs) have a large range of applications, such as pollutant removal and inductive heating. Some of these applications benefit from coating the IONPs with silica (SiO₂) to conserve their properties and/or prevent their aggregation; yet, the habitual synthesis methodologies require several steps, which limit their industrial scalability. In this work, we explore the capability to synthesize and stabilize oxidation-sensitive phases of IONPs via gas-phase flame synthesis as an alternative methodology that enables continuous operation. The addition of an inline quench gas nozzle—to avoid aggregation/agglomeration—and a coating nozzle is investigated to clarify their roles in contributing to the properties of the resultant coated IONPs. Three different quench and coating configuration heights above burner (HAB) are studied. The resultant synthesized Fe_xO_y|SiO₂ core–shell nanoparticles are characterized using (scanning) transmission electron microscopy ((S)TEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), elemental analysis, dynamic light scattering (DLS), Mössbauer spectroscopy, magnetometry, and energy-dispersive X-ray spectroscopy (EDX) from scanning electron microscopy (SEM). Results show that the synthesized nanoparticles presented a mixture of oxidation states—mainly magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) phases—and a narrow primary particle size distribution. Quenching the IONPs early decreased the nanoparticle agglomeration/aggregation up to one order of magnitude. Moreover, homogeneous coating was achieved in all cases. Increasing the coating thickness helped reduce oxygen diffusion to the iron oxide core of the coated IONPs, conserving more magnetite phase in the coated IONP cores. These insights allowed us to conclude that targeted coated IONPs can be successfully produced through gas-phase synthesis using a flame reactor. In the near future, the long-term stability of IONP properties will be explored using this inline coating.