Gas phase condensation of superparamagnetic iron oxide–silica nanoparticles – control of the intraparticle phase distribution

Abstract

Spherical, softly agglomerated and superparamagnetic nanoparticles (NPs) consisting of maghemite (γ-Fe₂O₃) and amorphous silica (SiO₂) were prepared by CO₂ laser co-vaporization (CoLAVA) of hematite powder (α-Fe₂O₃) and quartz sand (SiO₂). The α-Fe₂O₃ portion of the homogeneous starting mixtures was gradually increased (15 mass%–95 mass%). It was found that (i) with increasing iron oxide content the NPs’ morphology changes from a nanoscale SiO₂ matrix with multiple γ-Fe₂O₃ inclusions to Janus NPs consisting of a γ-Fe₂O₃ and a SiO₂ hemisphere to γ-Fe₂O₃ NPs each carrying one small SiO₂ lens on its surface, (ii) the multiple γ-Fe₂O₃ inclusions accumulate at the NPs’ inner surfaces, and (iii) all composite NPs are covered by a thin layer of amorphous SiO₂. These morphological characteristics are attributed to (i) the phase segregation of iron oxide and silica within the condensed Fe₂O₃–SiO₂ droplets, (ii) the temperature gradient within these droplets which arises during rapid cooling in the CoLAVA process, and (iii) the significantly lower surface energy of silica when compared to iron oxide. The proposed growth mechanism of these Fe₂O₃–SiO₂ composite NPs during gas phase condensation can be transferred to other systems comprising a glass-network former and another component that is insoluble in the regarding glass. Thus, our model will facilitate the development of novel functional composite NPs for applications in biomedicine, optics, electronics, or catalysis.