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 _x O _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.

Fig. 1. Reactor setup (left) and different nozzle configurations with spacings between the burner-nozzles marked (right).

Fig. 2. TEM images of the coated IONPs (Cases 0, 1, and 2).

Fig. 3. Cumulative particle size from DLS. Full lines refer to the experiments without coating, while dashed-dotted ones are for the experiments generating coated nanoparticles. Refer to online publication for colors.

Fig. 4. TEM images of the coated IONPs. The scale bar (white line) represents 10 nm. The thickness values correspond to the specific images and not the median for the entire sample.

Fig. 5. Mössbauer spectra recorded at 5 K in a magnetic field of 8 T applied parallel to γ-ray propagation direction, showing contributions of A-site (green), B-site Fe³⁺ (blue), and B-site Fe²⁺ (pink). Refer to online publication for colors.

Fig. 6. Mössbauer spectra recorded at 300 K, showing contributions of the iron ions on A- (green), B-sites (blue), and superparamagnetic nanoparticles (orange). Due to the stronger superposition and the deformation of the spectral structure by superparamagnetic relaxation, B-site contributions of Fe²⁺ and Fe³⁺ were not reproduced individually. Refer to online publication for colors.

Fig. 7. (a) M(H) magnetization curves recorded at 300 K and (b) zoom-in of (a). Magnetization values are normalized to the total sample mass. Refer to online publication for colors.

Fig. 8. Coercive fields H_C extracted from the M(H) curves recorded at 5 K (black) and 300 K (red). Refer to online publication for colors.

Fig. 9. High-field magnetization values M (9 T) extracted from the M(H) curves recorded at 5 K (black) and 300 K (red). Magnetization values are normalized to the total sample mass. Refer to online publication for colors.